You'll need a good non-stick saucepan for this one but it's really quick and easy and you don't need to make a separate sauce.
Boil the macaroni (with some chopped onion if you like) for about 8 - 10 minutes, until al dente. Drain into a collander.
Return to the non stick saucepan and stir in a generous helping of strong grated cheddar cheese.
Serve when the cheese has melted.
If you want to make it more interesting, fry some chopped onion, red pepper and mushrooms in the non stick saucpan, and cook the pasta separately. After you've drained the cooked macaroni, stir in the fried veggie mixture and then the cheese.