1/2 c. chopped onion
2 tbsp. butter
1 can mushroom soup
1/2 c. milk
1/2 c. drained chopped, canned tomatoes
2 c. cooked noodles
1 c. cooked peas
2 (7 oz.) cans tuna

In saucepan, cook onion in butter until tender. Stir in remaining ingredients. Pour in 2 quart casserole. Bake at 400 degrees for 20 minutes. Top with sliced cheese. Bake until cheese melts.