In a large stockpot or dutch oven over medium high heat, combine spaghetti, sausage, onion, tomatoes, basil, garlic and 4 1/2 cups water; season with salt and pepper, to taste.
Bring to a boil; reduce heat and simmer, uncovered, until pasta is cooked through and liquid has reduced, about 8-10 minutes. Stir in Parmesan.