Place a large skillet over medium-high heat. Add the crumbled sausage and minced onion. Cook for approximately 5 minutes, using a wooden spoon to break the sausage into small pieces. Drain any excess fat, then return the mixture to the skillet.
Incorporate the undrained diced tomatoes, tomato sauce, water, red pepper flakes, and uncooked penne. Stir thoroughly to ensure all ingredients are well combined, then bring the mixture to a boil.
Cover the skillet and reduce the heat to a simmer. Allow the pasta to cook for about 15 minutes, or until it reaches the desired tenderness.
Uncover and sprinkle with grated Parmesan cheese. Serve immediately, with additional cheese on the side if preferred.