Brown the ground beef in a large Dutch oven or stockpot over medium-high heat. Add the diced onion, bell pepper, salt, pepper, Italian seasoning, and crushed red pepper. Continue cooking until the beef is browned and the onions become translucent.
Stir in the garlic and cook for about one minute until fragrant.
Add the tomato sauce, diced tomatoes, and tomato paste. Stir until everything is well combined.
Once the sauce begins to bubble, stir in the mushrooms and zucchini. Cover the pot, reduce the heat to medium-low, and simmer for at least 30 minutes. Longer simmer times allow the flavors to deepen even more.
Serve immediately over your favorite pasta, or allow the sauce to cool completely before freezing.