Tuna Noodle Casserole

3 cups cooked egg noodles
1/4 cup chopped onion
2 tbl. butter or margarine
1 can cheddar cheese or mushroom soup
1 large can (or 2 small cans) tuna, drained and flaked
1 cup chopped celery
3/4 cup milk
2 tbl. flour
1/4 cup pimiento
1/4 cup parmesan cheese

Cook noodles according to package directions; drain and set aside. Meanwhile, in a saucepan cook celery and onion in butter or margarine till tender but not brown. Stir in flour and soup. Gradually stir in milk. Cook and stir till thick. Stir in tuna, pimento, and noodles. Turn into 1 1/2 quart casserole sprayed with non-stick cooking spray and sprinkle with cheese. Bake, uncovered, in a 375ºF oven for 20 to 25 minutes.