Bring a large pot of salted water to a boil. Add the pasta and cook according to the package directions until it is al dente. Before draining, reserve about 1/2 cup of the starchy pasta water. Drain the rest and set the pasta aside.

While the water boils, heat the olive oil in a large skillet over medium-high heat. Add the spicy Italian sausage. Use a wooden spoon to break the meat into small crumbles. Cook until the sausage is browned and no longer pink, then remove excess grease from the pan if necessary.

Lower the heat to medium. Add the diced onions to the skillet with the sausage. Sauté for about 3-4 minutes until the onions are translucent and soft. Stir in the minced garlic and cook for another 60 seconds until fragrant, being careful not to burn the garlic.

Pour in the crushed tomatoes and add the red pepper flakes if you want more spice. Season with a pinch of salt and pepper. Let the sauce simmer gently for about 10 minutes to allow the flavors to meld together.

Stir in the heavy cream and half of the grated Parmesan cheese. Continue to stir until the sauce is smooth and slightly thickened. The sauce should turn a beautiful light orange or pinkish hue.

Add the cooked pasta to the skillet. Toss everything together until the pasta is thoroughly coated in the sauce. If the sauce seems too thick, add a splash of the reserved pasta water. Garnish with the remaining Parmesan cheese and fresh parsley before serving.