disclaimer, again... i don't measure!
1 12 oz bag of pasta... (macaroni or penne) cooked according to directions.
(*i prefer to use dreamfield brand pasta... it's only got 5 net carbs per serving, so it's good if you're dieting or if you're diabetic.)
drain the pasta and set aside for a few minutes.
sautee about half a chopped onion (more or less depending on how big the onion is) in a decent amount of butter... enough so that you can use it for a roux when the onions are done.
make your roux by mixing in some flour, salt, and pepper... i love course ground black pepper in mine.
slowly add milk, stirring constantly til you get a white cream sauce that is slightly starting to thicken. before it gets too thick, add a bunch of grated cheese. keep stirring, adding milk as necessary because you don't want it too thick or stringy. (colby/jack works better than cheddar, because cheddar DOES get stringy!)
add in your noodles to the sauce, stir really good so that it gets all the pasta covered... then cover with additional cheese, cover, and bake until the pasta is hot all the way through. obviously, unless you have a large pan that can be used on stove-top AND in oven like i do, you may have to transfer it into a baking dish.
this is really yummy. i know a number of people who hate mac & cheese because the only kind they ever had was out of a box... then they tasted it homemade and LOVED it. and it never comes out tasting the same way twice, because you don't measure anything ;)
when re-heating leftovers, be sure to add more milk.
oh, and another little tip... if you've made, for instance, a beef or pork roast, you can add some of the broth in while you're making the sauce... just a few tablespoons or so, for additional flavor.
