1/3 c. butter
1 lg. onion, finely chopped
2 (8 oz.) cans tomato sauce
1/4 lb. mushrooms, sliced
1 lg. garlic clove, minced
1/2 tsp. dried oregano
1/2 tsp. dried basil
1 (8 oz.) pkg. spaghetti, cooked al dente and drained
1 (4 oz.) pkg. pepperoni, thinly sliced
8 oz. Mozzarella cheese, shredded (about 2 c.)
3 1/2 oz. Swiss cheese, shredded (about 1 c.)

Preheat oven to 350 degrees. Lightly grease a 13 x 9 x 2 inch baking dish. Melt butter in frying pan; add onion and cook until soft, about 5 minutes. Remove from pan and set aside. Combine tomato sauce, mushrooms, garlic, oregano, basil and cooked onions. Mix thoroughly. Arrange cooked, drained spaghetti in bottom of prepared dish. Top with half the tomato sauce mixture, dot with half the pepperoni and sprinkle with half the cheeses. Repeat layering with remaining tomato sauce mixture, pepperoni and cheeses. (Can be prepared ahead to this point and refrigerated.) Bake until heated through, about 25 to 30 minutes. Serve immediately. Serves 6 to 8.