1 med. onion, diced
1/3 c. melted butter
37 oz. tomato sauce
1/2 tsp. basil
1/2 tsp. oregano
1 (12 oz.) pkg. spaghetti, cooked & drained
1 1/2 c. grated Swiss cheese
3/4-1 lb. Mozzarella cheese, grated
1 (8 oz.) pkg. pepperoni
8 oz. can mushrooms, drained

Simmer onions in butter, add sauce and spices. Simmer 15 minutes. Cook and drain spaghetti. Put 1/2 of spaghetti noodles in bottom of 9"x13" pan, layer of cheese, pepperoni, half of mushrooms and one half of the sauce mixture. Repeat layers in same order. Bake at 350 degrees for at least 45 minutes if casserole has been refrigerated before baking.