Thaw and drain (press dry in paper towel) the spinach; set aside.
Hard-boil 2 or 3 eggs before starting; set aside. (Peel when cool enough to handle.)
Grate the cheese; set aside.
Finely chop up an onion. Zap it for a minute or two in a microwave to soften it a bit. Set aside.
Slice a tomato, set aside.
Now you're ready to bring it all together.
Bring lightly salted water to boil in a large saucepan.
While the water is coming to a boil, melt the butter in a small saucepan over low heat. When melted, stir in the onion, cook for about 3 to 4 minutes, then stir in the flour and mustard powder to make a paste. Cook over low heat for 2 minutes, stirring all the time. Don't let it brown.
Add macaroni to boiling water in large saucepan and cook according to package instructions -- probably 8 to 10 minutes. Don't overcook; leave a bit of bite to the macaroni.
While the macaroni is cooking, whisk the milk a little at a time into the butter and flour paste. Whisk each addition until smooth. When all the milk is in, add the spinach and continue to simmer the sauce (whisking it regularly) until it thickens as much as it looks like it is going to.
When the macaroni is cooked, put the colander in the sink and drain the macaroni in it. When drained, put the macaroni back in the large saucepan.
When the sauce has fully thickened, stir in two-thirds of the cheese. Season with salt and pepper to taste. Pour over the macaroni into the large saucepan, and stir to cover all the macaroni with the sauce.
Start heating oven to 200 C / 400 F / Gas Mark 6.
In a buttered baking dish, space out the whole, peeled, boiled eggs. Pour the macaroni and cheese mixture on top of this.
Transfer it all to the baking dish, and smooth the top level (use the wooden spoon to scrape the all remaining sauce out of the pans.) Top with the sliced tomatoes.
Then, crush the chips roughly (aka crisps in the UK), and sprinkle them over the top. Cover with the remaining ⅓ of the cheese.
Cook uncovered in oven for 20 to 25 minutes; it should be golden and bubbling when done.
Notes
You will need the following equipment: measuring cup (jug), 1 large and 1 small saucepan, chopping / cutting knives, colander, wooden spoon, whisk, 1 ½ quarter (1 ½ litre) baking dish (larger is fine), oven gloves.Options: Instead of frozen spinach, you can cook up fresh. Drain after.Stir chopped, cooked smoky bacon into the macaroni and sauce mixture before pouring into the baking dish.