Fill a medium to large pot one-half full of water. Add 1 tablespoon salt, and bring the water to a full boil. Add the 12 oz. of pasta, and bring to a boil. Cook for 7 to 10 minutes, until al dente (chewy--not mushy). While the pasta is cooking, prepare your casserole cheese sauce. Melt 2 tablespoons butter in a saucepan on the stove. Remove from heat and stir in 1/4 cup flour an stir vigorously, until the two are blended into a roux. Add 12 oz. (1 1/2 cups) grated Parmesan cheese, 1 1/2 cups milk (I use skim milk), and 1/4 teaspoon salt. Now cook and stir continuously for a minute or two, and then occasionally. The sauce should be thick and smooth about the same time that the pasta is cooked. Drain the pasta in a colander in the sink and place the pasta back in the pot. Stir in 1 additional cup milk, (2) 6 oz. cans chunk light tuna, drained, and 1/2 cup frozen green peas. Mix thoroughly. transfer the mixture into an large oven-proof dish, which has been sprayed with cooking spray. Bake in an oven that has been preheated to 350 degrees for 35 minutes. Remove from oven, and let stand about 5 minutes before serving. Serve to your grateful guests! This recipe makes a large casserole, so it is very economical; also, your kids will love it, because it is a variation on macaroni and cheese! If you don't have a large family, you may freeze the leftovers, and avoid cooking a meal later! A real winner in every way!!!