In a large skillet over medium-high heat, combine sausage, onions, carrots, and garlic. Cook until the sausage has browned and the vegetables have softened, about 5 to 7 minutes. Drain if desired.
Meanwhile, to the slow cooker, add diced tomatoes and their juice, tomato sauce, mushrooms, Italian seasoning, basil, fennel, and salt and pepper to taste (I like 1 teaspoon salt and ½ teaspoon pepper). Stir in water and sausage mixture.
Cover and heat on HIGH for 4 hours or LOW for 8 hours. During the last 10 minutes of cooking time, stir in the pasta, cover, and continue to cook until softened. Serve with mozzarella cheese and fresh herbs if desired.
Lasanga noodles: Put those leftover lasagna noodles from your pantry to work in this Lasagna Soup. Just break the dry noodles into bite-sized pieces. Or, substitute any other small pasta (radiatore and rotini are my favorites here).