Step 1: In a large, deep skillet or Dutch oven, heat olive oil over medium-high heat. Add the Italian sausage (casings removed) and break it up with a spoon. Cook until deeply browned and cooked through, about 5-7 minutes. Drain any excess fat, leaving about a tablespoon in the pan for flavor. Transfer the cooked sausage to a plate and set aside.
Step 2: Reduce the heat to medium. Add the chopped onion to the skillet and sauté until softened and translucent, about 3-4 minutes. Add the minced garlic and red pepper flakes (if using) and cook for another minute until fragrant, being careful not to burn the garlic.
Step 3: Pour in the chicken broth or white wine, scraping up any browned bits from the bottom of the pan. Add the crushed tomatoes and dried Italian seasoning. Stir well, bring the sauce to a gentle simmer, then reduce the heat to low, cover, and let it cook for at least 15-20 minutes to allow the flavors to meld.
Step 4: While the sauce simmers, bring a large pot of heavily salted water to a rolling boil. Add the rigatoni pasta and cook according to package directions until al dente. Before draining, reserve about 1 cup of the starchy pasta water. Drain the remaining pasta.
Step 5: Stir the heavy cream and 1/2 cup grated Parmesan cheese into the simmering tomato sauce. Mix until the cheese is melted and the sauce is smooth and creamy. Season with salt and black pepper to taste. If the sauce seems too thick, add a splash of the reserved pasta water until it reaches your desired consistency.
Step 6: Add the cooked Italian sausage back into the sauce. Add the drained al dente rigatoni to the skillet with the sauce. Toss everything together until the pasta is thoroughly coated. If needed, add another splash of pasta water to loosen the sauce. Serve immediately, garnished with fresh chopped parsley and extra grated Parmesan cheese.