Combine ground sausage, onions, and garlic in a large pot or skillet with tall sides. Cook over medium heat until sausage is cooked through, 5 to 8 minutes. Drain and discard grease.
Stir pasta sauce, water, and Italian seasoning into the pot; bring to a boil. Stir in spaghetti noodles, return to a boil, and cook, stirring occasionally, until noodles are cooked through and sauce has thickened, 17 to 20 minutes.
Serve topped with grated Parmesan cheese.