Tafelspitz is an Austrian boiled dinner, consisting of beef simmered with root vegetables.
It takes its name of "Tafelspitz" from the cut of beef used, which is also called Tafelspitz. This means, literally, "table tip."  In practice in Austria, many people now use other cuts of beef as this cut is in high demand for this particular dish, and therefore expensive.
Tafelspitz can be served with some form of potato such as sautéed, roasted, rosti or julienned, a relish of minced apple mixed with horseradish, a breadsauce with chives and egg in it, and sometimes sour cream as well.
To make Tafelspitz, you simmer the meat for 3 to 4 hours in unsalted water, along with some beef marrow bones (recipes seem to indicate they should be cut into 2 inch / 5 cm slices) and peppercorns. The layer of fat on the Tri-Tip should be left on during cooking. Skim the broth occasionally as foam occurs -- you want to end up with a clear broth. One hour before the meat is tender, add the prepared root vegetables (washed, peeled, bite-sized cubes.) These vegetables can include carrots, leeks, celeriac, onion, turnip, parsnip, etc. The onions are prepared in a special way: you cut the onion into halves, leaving the skin on, and in a frying pan with no fat, brown each exposed surface first before adding -- this gives the broth some colour.
When the vegetables are cooked, remove the meat and bones, discarding the bones. Flavour the broth with seasonings such as salt, pepper, and lovage. (No salt should have been added before this point.) Then strain the broth, keeping the vegetables.
Serve slices of the beef with a small drizzle of broth on each slice (about 1 tablespoon), the vegetables and the other side sauces mentioned above.
At the Plachutta restaurant in Vienna, Tafelspitz is served as three separate courses. The broth with the vegetables in it is served as a soup (dumplings or noodles may be added.) Then, the marrow from the marrow bones is served with toasted pieces of rye to spread it on. Finally, the main course of slices of the beef with about a tablespoon of the broth on it, with two of the side sauces mentioned above, potatoes, and an additional vegetable, such as creamed spinach.
 The Tafelspitz is a pointy-shape cut from the top of a hind leg (aka Sirloin Bottom) of cow. In North American butchering, the Sirloin Bottom can be divided into three parts, of which the Tri-Tip is one. A Tri-Tip cut is the closest cut in North America to a Tafelspitz cut. In the UK or Australia, try a silverside cut.