Combine beef, onion, and garlic in a large skillet and cook until meat is browned; drain.
In a 4-quart crock pot, combine tomatoes, tomato paste, mushrooms, bay leaf, Italian seasoning, salt, and black pepper.
Stir in the meat mixture.
Cover and cook on LOW for 8 to 10 hours, or on HIGH for 4 to 5 hours.
Stir in the chopped sweet peppers.
Discard bay leaf.
Spoon sauce over cooked spaghetti, garnish with parmesan cheese and serve.
Notes
Slow Cook: Simmer the sauce slowly and steadily for deeper flavors.
Tomatoes: Use quality canned tomatoes for the best flavor.
Mushrooms: The recipe uses canned mushrooms, but you can use 6 to 8 ounces of fresh sliced mushrooms if you prefer.
Adjust Seasonings: Taste for salt, pepper, and seasonings, and tweak as needed.
Swap Ingredients: Substitute beef with other meats or lentils. Replace mushrooms with preferred veggies. Try different herbs or add heat with red pepper flakes.
Stovetop Version: Brown the meat and aromatics in a large pot; add everything else to the pot and simmer for 1.5 and up to 4 hours.
Storage: Once the sauce has cooled, store it in an airtight container and keep it in the refrigerator. It should last for up to a week.
Adapted from Better Homes and Gardens New Cook Book, Sixteenth Edition