VERY basic recipe from my Better Homes and Gardens cookbook, but I like it because it's extremely quick and simple and pretty easy to vary.
6 oz elbow macaroni (1 -1/2 cups)
3 tablespoons butter
1/4 cup finely chopped onion (I sometimes omit this)
2 tablespoons all-purpose flour
1/2 teaspoon salt
2 cups milk
2 cups cubed American cheese (I use 1/2 lb shredded mix of cheddar, monty-jack, pepperjack, whatever)
1 medium tomato (I omit this)
Preheat oven to 350deg . Cook macaroni accordin to package directions; drain. For cheese sauce, in a saucepan melt butter. Cook onion in butter till tender but not brown (I sometimes omit the onions). Stir in the flour, salt, and pepper. Add milk all at once; cook and stir until thickened and bubbly. Cook and stir 1 to 2 minutes more. Add cubed cheese (shredded, in my case); stir till melted.
Stir macaroni into cheese sauce. Turn into a 1-1/2 qt casserole. Arrange tomato slices on top; sprinkle with a little salt (I omit the tomatoes). Bake in 350deg oven 30 to 35 minutes or until heated through (in my case, slightly browned around the edges).