Slow Cooker Zucchini Soup

Name: Slow Cooker Zucchini Soup
Prep time: 30 min
Cook time: 4 h 15 min
Total time: 4 h 45 min
Cuisine: American
Ingredients: 1.5 pounds sweet Italian sausage; 2 cups 1/2-inch pieces celery; 2 pounds zucchini, cut into 1/2-inch slices; 2 (28 ounce) cans diced tomatoes; 2 green bell peppers, cut into 1/2-inch slices; 1 cup chopped onion; 2 teaspoons salt; 1 teaspoon white sugar; 1 teaspoon dried oregano; 1 teaspoon Italian seasoning; 1 teaspoon dried basil; 0.25 teaspoon garlic powder; 6 tablespoons grated Parmesan cheese, or to taste