In a Dutch oven, combine the spaghetti, sausage, grape tomatoes, basil, onion, garlic, and water.
Bring it all to a boil over medium-high heat. Reduce the heat and let it simmer, uncovered, for 8-10 minutes. Stir occasionally to keep the pasta from sticking.
Once the pasta is al dente, stir in the Parmesan cheese, salt, and pepper. The cheese will melt into the sauce, making it rich and creamy.
If you like a bit of heat, sprinkle in the crushed red pepper flakes. Serve with an extra sprinkle of Parmesan and fresh basil if desired.