Cheddar Tuna Casserole

1/2 c chopped onion
2 tbsp margarine
1 can cream of celery soup
1/3 c milk
8 oz egg noodles, cooked and drained
1 6 1/2 oz can tuna, drained and flaked
4 oz pkg shredded cheddar cheese
1/4 c grated parmesan cheese

Saute onion in margarine; add soup and milk. Stir in remaining ingredients except 1/2 cup shredded cheddar cheese. Pour into 1 qt casserole. Bake at 350 for 30 minutes. Top with remaining cheddar cheese and continue baking until melted.