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

Preheat oven to 350 degrees. Lightly grease 9"x13" baking dish. Melt butter in heavy medium skillet. Add onion and cook over medium high heat until translucent, about 6 minutes. Remove from skillet and set aside. Combine tomato sauce, mushrooms, garlic, oregano, basil and onion; mix thoroughly.

Arrange spaghetti in bottom of prepared pan. Top with half of tomato sauce mixture; dot with half of pepperoni and sprinkle with half of cheeses. Repeat layering with remaining tomato sauce mixture, pepperoni and cheese. May be prepared ahead to this point and refrigerated. Bake until heated through, about 25 minutes. Serve immediately.