Like Caesar Salad, there are so many variations of Steak Diane, and so many people saying "well, the way I like to make it....", that it's difficult to know what's authentic anymore.
In general, it's a tender, boneless beef steak, usually cut from beef tenderloin, and generally about 1/2 inch (1 cm) thick, whether "naturally" that thick or because it was pounded to reduce the thickness.
It is sautéed. While a few versions don't flambé it afterwards, many people now expect it to be flambéed in cognac with a sauce that has mustard in it. Flambéing concentrates and carmelizes the juices.
In restaurants, it is traditionally made in a chafing dish at the table.
Many people that sauce that is made in the pan with the steak is "Sauce Diane." It is not; that is a separate sauce, for game.
It has nothing to do with the definition of "Diane" in classical French cooking, as there is no game involved (Diane being Diana, the Greek goddess of the hunt.)
Perhaps the best guess is that it originated with Beniamino Schiavon (aka "Nino"), from Padua, Italy. He worked at the Drake Hotel in New York. The earliest print references found to date, dating from 1949, not only point to him, but talk about the recipe being wheedled out of him by no less a person than Perle Mesta. Not just any old society queen, Mesta also was the American ambassador to Luxembourg from 1949 to 1953.
By early 1951, writers were using it as a throw-away line, expecting people to have heard of it: "Certainly, you could not feed dogs of such lofty cognomen anything less than caviar and Steak Diane or Steak Poivrade." 
By 1953, Jane Nickerson reported (in the New York Times, 25 January 1953) that the Drake Hotel, the Sherry-Netherland Hotel and the Colony Restaurant were all claiming to have the best Steak Diane, though Nino went further and claimed to have introduced it to the United States. In the same article, she gave the Colony's recipe which included butter, salt, pepper, chives, Worcestershire sauce -- but no alcohol or mustard.
It might appear, though, that the version which truly become popular and which popularized the dish was one published by Swift & Company meat company of Chicago in 1954, which called for mustard.
By the 1960s, it was de rigeur in trendy American restaurants.
But, finally, throwing a wrench in all the above theorizing, is the Café de Paris in London, England. On their web site (as of July 2006), they claim that in the 1930s, "The Aga Khan became a frequent visitor as did Lord and Lady Mountbatten, who nearly always ordered the same dinner of 'a dozen and a half oysters and steak Diane'."
On their websites in 2006, the Chicago Meat Authority Company and the Minnesota Beef Council also tossed another theory into the ring: that it was invented in Rio de Janeiro, Brazil, at the Copacabana Palace Hotel, but they give no backup for their claim.
 Ruark, Robert C. At Least Something You Can Holler. Charleston, West Virginia: The Charleston Daily Mail. Tuesday, 20 February 1951. Page 6.
Literature & Lore
-- Kilgallen, Dorothy. Voice of Broadway column. Olean, New York: Olean Times Herald. Olean, New York. Thursday, 7 April 1949. Page 17.
"Perle Mesta's banquets in Luxembourg feature Steak Diane as the piece de resistance. She wheedled the recipe from Nino of the Drake Hotel."
-- Kilgallen, Dorothy. Voice of Broadway. Olean, New York: Olean Times Herald. Wednesday, 2 November 1949. Page 12.
"The Gray Merrills (Bette-than-ever Davis) devouring 'Steak Diane' at Herman Billingsberg's [Ed.: the Stork Club, owned by Billingsberg] . . . "
-- Winchell, Walter. In New York. Charleston, West Virginia: Charleston Daily Mail. Friday, 16 January 1953. Page 12.
"What's behind a recipe? Where do those taste-tempting food photographs originate? A trip behind the scenes in a food company to answer these questions brings us to the busy gleaming test kitchens of Swift & Company in Chicago where a capable staff of home economists produce a steady flow of new recipes, products and information for homemakers.
Millions of homemakers from coast to coast know Martha Logan, home economist for Swift & Company. The dynamic person behind the name is Mrs. Beth Bailey McLean, director of the company's home economics division....
Recipe testing and development is a fascinating phase of the Martha Logan activity....
For an example, let's follow the development of a recipe for Steak Diane. After watching the preparation of Steak Diane by a New York hotel chef, Mrs. McLean tried it in her own kitchen, modifying the procedure to the homemaker's equipment and service. She served the Steak Diane and her family enthusiastically approved. Mrs. McLean arrived at the Martha Logan test kitchens the following day and told the home economist in charge of recipe development about her discovery.
With suggestions from Mrs. McLean, the Martha Logan [staff] then prepared many variations of ingredients to select the exact combination for the finest-flavored dish. Several home economists checked the preparation until the final combination was ready for approval from the entire Martha Logan staff. And so, a new recipe is born....
Here is the recipe for Steak Diane that was tested, enthusiastically approved and added to the recipe materials put out by the company.
Yield; 4 Servings
4 boneless top sirloin or club steaks, cut thin
1/4 cup salad oil
1 clove garlic, peeled and sliced
1/4 cup butter
1 teaspoon dry mustard
1/2 teaspoon salt
1/4cup chopped parsley
2 teaspoons lemon juice
1 teaspoon Worcestershire sauce
Freshly ground black pepper
Trim most of fat off steaks. Combine salad oil and garlic. Let stand at least 5 minutes. Brush on both sides of steaks. Stir together butter, dry mustard, salt and parsley. Melt mixture on heated grill. Place steaks on this mixture. Use a bowl or small pan to catch any sauce that drains off.
After about 5 minutes' cooking, turn steaks and cook 5 minutes more and pour juice, Worcestershire sauce and black pepper on grill. Stir and swish steaks in sauce. To serve, lift steak onto hot plate, add a hot grilled bun and pour over steak some of the extra sauce that has drained into catcher pan."
-- Loofe, Marion. Trip Behind Scenes Answers Your Questions On Recipes. Eureka, California. Humboldt Standard. Thursday, 22 April 1954. Page 13.
"When you get right down to it, the food and booze aren't much different than at any other good eatery: indeed, my pounded-down steak was good but a shade below the Steak Diane served by Nino Schiavon at the Dark Room. It's just that the chant started a long time ago at the Colony and if you didn't go there, my dear, you just didn't count."
-- Heimer, Mel. New York. Lancaster Eagle Gazette. Lancaster, Ohio. Saturday, 10 August 1963. Page 6.
"THE DINNER was superb -- a perfect gazpacho, neither too chilled nor warm, a perfectly prepared Steak Diane, sauteed in butter at the table, flamed with Courvoisier and flanked with dark, separated wild rice."
-- Bolton, Whitney. Best Eating at Fair [Ed.: New York World's Fair 1964]. Elyria, Ohio. The Chronicle Telegram. Saturday, 1 August 1964. Page 32.
STEAK DIANE: A dish which is even more spectacular than Boeuf Strogonoff is Steak Diane. If it appears on the menu of a first-class restaurant, order it if you want, a lesson in its cooking. The head waiter will bring in your table a trolley on which are a spirit lamp, a copper frying pan, a jug of rich stock, a chopped shallot, a bottle of Worcester sauce, butter, chopped parsley and salt and pepper. Also for four servings, four steaks beaten very thinly to twice or more their original size will do this. First the waiter will pour a dessert spoon of the sauce into the frying-pan and evaporate it. This will remove the moisture but leave the flavour of the herbs and spices behind. Next, into the pan goes a biggish lump of butter in which the chopped shallot is cooked. Then the steaks are added, quickly cooked on both sides, seasoned to taste and placed on a heated dish. To the frying pan, are added another piece of butter, a tablespoon of Worcester sauce and a little of the rich stock. These are swirled round the pan, brought to the boil, poured over the steaks which are then sprinkled with parsley and served at once. I have seen head waiters melt butter in another frying pan on the trolley and add cold cooked French beans first turned in flour, to it, then toss them about to brown the flour and heat through -- but that perhaps, is a little too much to expect of a beginner. Just cooked beans turned in butter, and fried potatoes would be excellent with the steaks."
-- Burke, Helen. The secret of Strogonoff. Kingston, Jamaica. The Daily Gleaner. Thursday, 13 August 1970. Page 22.
Beef DishesAllumettes de Palais de Boeuf; Beef Dishes; Beef Stroganoff; Beef Wellington; Bistecca alla Fiorentina; Cincinnati Chili; Ground Meat Dishes; Irish Spiced Beef; Meat Dishes; Morcon (Philipino); Ossobuco; Shit on a Shingle; Sliders Hamburgers; Steak Diane; Tournedos Rossini; White Collops; Yankee Pot Roast
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