Cook the Pasta: Bring a large pot of salted water to a boil. Add the pasta and cook until just tender. Stir once or twice so it does not stick. Before draining, save about half a cup of pasta water. Drain and set aside.
Brown the Sausage: Heat olive oil in a large pan over medium heat. Add the Italian sausage and break it into small pieces. Cook until browned and cooked through. This step builds most of the flavor.
Add Onion and Garlic: Stir in the diced onion and cook until soft and lightly golden. Add the garlic and cook for about 30 seconds until fragrant. Sprinkle in red pepper flakes if you want heat.
Pour in the Cream: Lower the heat slightly and slowly pour in the heavy cream. Stir well so it blends with the sausage and onion. Let it simmer gently for a few minutes until slightly thick.
Season the Sauce: Add salt and pepper to taste. Keep the seasoning light at first since the cheese adds salt later. Stir often to keep the sauce smooth.
Combine Pasta and Sauce: Add the cooked pasta directly into the pan. Toss until every piece gets coated. If the sauce feels thick, add a splash of the reserved pasta water.
Finish with Cheese and Basil: Sprinkle in the Parmesan cheese and stir until melted. Turn off the heat and fold in the chopped basil. Taste once more and adjust seasoning if needed.
