Cook the macaroni in a medium to large pot that is half-full of boiling water, salted with 1 tablespoon of salt. Place 7 oz. of macaroni in the boiling water. It will take about 8 minutes for the macaroni to cook to a consistency that is al denti (chewy, but not mushy). It will continue to cook when baked in the casserole. Meanwhile, prepare your tomato-cheddar cheese sauce. In a medium-sized sauce pan, melt 1 tablespoon butter or margarine, and add 1/2 cup chopped onion. Saute the onion until clear. Add a 10 3/4 oz. can of condensed cream of mushroom soup, 1/2 cup milk, 2 cups shredded cheddar cheese, and a 14.5 oz. can of diced tomatoes (drained). Mix thoroughly. Now, place the cheese sauce over low heat, and cook and stir until the mixture is smooth and cooked through. Move the cheese sauce to a "warmer-type" burner until your macaroni is done, and stir it occasionally. When the macaroni is done, pour it through a colander to remove all the water, and return the macaroni to its original pot. Stir in the cheese sauce gently until completely mixed. Spray an 8-inch by 10-inch Pyrex dish with cooking spray, and add the prepared macaroni to the dish. Place the dish of macaroni in an oven that has been preheated to 350 degrees. Bake for 25 to 30 minutes, or until done to your liking. This is a favorite recipe in my family. Kids love it,