In a large saucepan or skillet, brown meat, onion, green pepper and celery, drain off excess grease, return to pot. Add mushrooms, diced tomatoes, tomato sauce, garlic powder, oregano, bay leaf, salt and pepper. Simmer 15-20 minutes. Remove bay leaf before serving.
In a large pot, bring 4-6 quarts of water to a boil, add salt if desired. Cook spaghetti according to package instructions.
Serve sauce over spaghetti or pasta. Top with grated Parmesan cheese.