Bring 2 cups water and 1 teaspoon salt to boil. Add noodles and cook until al dente, about 8 minutes. Drain well and rinse in cold water to stop the cooking.
Meanwhile, in a Dutch oven or large saucepan over medium-high heat, add sausage, onions, carrots, and garlic. Cook until the sausage has browned and the vegetables have softened, about 5 to 7 minutes. Drain if desired.
Add water, 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 sausage mixture and bring to a boil.
Reduce heat, cover, and simmer until flavors have blended, about 15 to 20 minutes. Stir in drained pasta until heated through, about 5 minutes. Serve with mozzarella cheese.
Notes
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).
Italian sausage: Ground beef may be substituted for the Italian sausage.
Mushrooms: The mushrooms are optional and for true mushroom lovers only! Don't feel obligated to put them in if you don't love them.
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.
Yield: This Lasagna Soup recipe makes about 12 cups of soup, enough for 6 generous servings, 2 cups each.
Storage: Store leftovers covered in the refrigerator for up to 4 days.
Freezer: For best results, omit the pasta before freezing. When it’s time to reheat the soup, boil the noodles separately and add them to the pot while it's reheating.