Smothering pasta in a flour-based sauce is not the kind of thing that Italians would do. But macaroni cheese is one of those dishes -- curry is another -- for which I get cravings.
Sometimes, I bake it. Baking dries up the sauce a little, and makes the pasta rather sticky -- but that can be what you want. Simply finishing the dish under the grill retains all the sauce and ensures that the pasta remains separate.
Heat up the water for the pasta.
Gruyere, Comte and Cantal are all good cheeses to use. I like a strong Cheddar best -- about 3 heaped tbsps for each person. After grating it, grate a little nutmeg on top. Grate also about 2 tbsps of Parmesan.
Warm a gratin dish. When the pasta goes in, it should not cool down.
You can make the sauce in the eight to 10 minutes it takes the pasta to cook. I use penne; conchiglie also work well, as do most shapes other than spaghetti or other ribbon types. About 100 g for each person, thrown into the boiling water with plenty of salt (about a tsp for each litre of water), is enough in this rich dish.
I like a generous quantity of cheesy bechamel: about as much as will be thickened by a generous dstsp of flour for each person. (You will need about 300 to 350 ml of milk for each serving, in other words.) Stir the flour into enough melted butter (about 20 g for each dstsp) to make a roux that is the texture of damp sand, and cook it over a gentle heat for about a minute -- long enough for some of the floury taste to be cooked out, but not long enough for the butter to brown. Turn up the heat to medium, and add the milk a few splashes at a time, incorporating each before pouring in the next. (If you have cream, you might like to use that instead of about a third of the milk.) The sauce should be of a coating consistency, but not stiff. When it is bubbling, stir in the cheese (with the nutmeg) until melted, and turn off the heat. Grind in pepper if you like. You do not need salt.
Drain the pasta. Stir it into the sauce; or, if the sauce is not sitting in a large enough pan, stir the pasta and sauce together in the pasta pan. Tip the mixture into the warmed gratin dish, sprinkle the Parmesan on top, and put the dish under the grill. (I remove the rack from the grill pan and put the dish in it, for easy manoeuvrability.) Watch out: Parmesan burns very easily.