Bring a large pot of salted water to a rolling boil. Add the rigatoni and cook according to package directions until al dente. Reserve 1 cup of pasta water before draining. Drain the rigatoni and set aside.

While the pasta cooks, heat the olive oil in a large skillet or Dutch oven over medium-high heat. Add the Italian sausage (casings removed) and break it up with a wooden spoon. Cook until browned and cooked through, about 5-7 minutes. Drain excess fat if necessary.

Add the chopped onion to the skillet with the cooked sausage. Sauté for 3-4 minutes until softened. Add the minced garlic and red pepper flakes (if using) and cook for another minute until fragrant, being careful not to burn the garlic.

Stir in the crushed tomatoes, chicken broth, dried oregano, and black pepper. Bring the sauce to a gentle simmer, then reduce heat to low, cover, and let it cook for 10-15 minutes, allowing the flavors to meld. Stir occasionally.

Remove the lid and stir in the heavy cream. Add the fresh spinach, stirring until it wilts into the sauce, about 2-3 minutes.

Add the cooked rigatoni and grated Parmesan cheese to the skillet with the sauce. Toss everything together until the pasta is evenly coated. If the sauce seems too thick, add a splash or two of the reserved pasta water until desired consistency is reached.

Serve immediately, garnished with extra Parmesan cheese and fresh chopped basil.