Cook the Pasta:
Bring a large pot of salted water to a boil. Add the penne pasta and cook according to package instructions until al dente. Reserve 1 cup of pasta water, then drain the pasta and set aside.

Brown the Sausage:
In a large skillet over medium-high heat, add the olive oil. Remove the sausage from its casing (if applicable) and add it to the skillet. Cook, breaking it up with a spoon, until browned and cooked through, about 5-7 minutes.

Sauté Aromatics:
Add the chopped onion to the skillet with the sausage and sauté until translucent, about 3 minutes. Stir in the minced garlic and red pepper flakes (if using), cooking for another 1-2 minutes until fragrant.

Create the Creamy Sauce:
Reduce the heat to medium. Pour in the heavy cream, stirring to combine. Allow the mixture to simmer for 2-3 minutes. Gradually add the grated Parmesan cheese, stirring continuously until the cheese has melted and the sauce is smooth. Season with salt and freshly ground black pepper to taste.

Combine Pasta and Sauce:
Add the cooked pasta to the skillet, tossing to coat evenly with the sauce. If the sauce is too thick, gradually add some of the reserved pasta water until the desired consistency is reached.

Serve:
Divide the creamy Italian sausage pasta among serving plates. Garnish with chopped fresh parsley and additional Parmesan cheese, if desired. Serve hot and enjoy!