In a large pot, bring salted water to a boil. Add the penne pasta and cook according to package instructions until al dente, about 8-10 minutes. Drain and set aside, reserving a little pasta water.

In a large skillet over medium heat, add the spicy Italian sausage, breaking it apart with a wooden spoon. Cook until browned and fully cooked, about 5-7 minutes. Remove from skillet and set aside.

In the same skillet, add olive oil. Once hot, add the chopped onion and sauté for 3-4 minutes until translucent. Add minced garlic and cook for an additional minute.

Pour in the crushed tomatoes, and stir in oregano, red pepper flakes, salt, and pepper. Let the sauce simmer for about 10 minutes.

Return the cooked sausage to the skillet and stir into the sauce. Pour in the heavy cream and mix until fully incorporated. Let it simmer for another 2-3 minutes.

Add the cooked penne pasta to the skillet and toss until evenly coated with the sauce. If the sauce is too thick, add a splash of reserved pasta water.

Stir in the grated Parmesan cheese until melted and creamy. Serve hot, garnished with fresh basil leaves.