1 pound ground Italian sausage
1 large onion, chopped (about 2 cups)
3 carrots, peeled and diced (about 1 cup)
4 cloves garlic, minced
2 (14.5 ounce) cans Italian-style diced tomatoes, undrained
1 (15 ounce) can tomato sauce
8 ounces sliced mushrooms, optional
1 teaspoon Italian seasoning
½ teaspoon dried basil
½ teaspoon fennel seeds
Salt and freshly ground black pepper
2 cans water
1 cup rotini or other small pasta
2 cups mozzarella cheese, for topping
In a large skillet over medium-high heat, combine sausage, onions, and carrots. Cook until the sausage has browned and the vegetables has softened, 5 to 7 minutes, breaking up any large pieces of sausage with a wooden spoon. Stir in garlic until fragrant, about 30 seconds.
Meanwhile, add diced tomatoes and their juice, tomato sauce, mushrooms, Italian seasoning, basil, fennel, 1 teaspoon salt, and ½ teaspoon pepper to the slow cooker. 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 on the side.