In a large skillet cook sausage, onion, and garlic over medium-high heat until sausage is browned, using a wooden spoon to break up sausage as it cooks. Drain off fat.
In a 3-1/2- or 4-quart slow cooker combine spaghetti sauce, mushrooms, and basil. Stir in sausage mixture. Cover and cook on low-heat setting for 3 hours. If desired, stir in sweet pepper. Cover and cook on low-heat setting for 15 minutes more.
To serve, spear vegetable dippers and/or bread pieces with fondue forks and dip into the fondue, swirling to coat.