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.
Italian sausage: Ground beef may be substituted for the Italian sausage. In a pinch, you could also chop up Italian sausage links.
Mushrooms: Unless it's a Stuffed Mushrooms recipe, I feel like mushrooms are always optional (people either love them or hate them). Don't put them in if it will take away your joy. It's as simple as that.
Italian seasoning: To make your own homemade Italian seasoning, combine equal parts of dried basil, marjoram, oregano, rosemary, and thyme. Store extra seasoning covered in the pantry for up to 6 months.
Lasagna 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 (radiatori and rotini are my favorites here).
Yield: This Slow Cooker Lasagna Soup recipe makes about 12 cups of soup, enough for 6 servings, 2 cups each.
Storage: Store leftovers covered in the refrigerator for up to 4 days.