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, then drain the rigatoni and set aside. Drizzle with a little olive oil to prevent sticking.

While the pasta cooks, heat 1 tablespoon of olive oil in a large deep skillet or Dutch oven over medium-high heat. Add the Italian sausage, breaking it apart with a wooden spoon. Cook, stirring occasionally, until deeply browned and cooked through, about 6-8 minutes. Drain off any excess grease and transfer the sausage to a plate, leaving a tablespoon of drippings in the skillet if desired.

Reduce the heat to medium. Add the diced yellow onion to the skillet and cook until softened and translucent, about 4-5 minutes. Add the minced garlic and dried oregano, cooking for another minute until fragrant, being careful not to burn the garlic. Add red pepper flakes now if using.

Pour in the crushed tomatoes and chicken broth. Bring the mixture to a gentle simmer, then reduce heat to low, cover, and let it cook for 10 minutes, allowing the flavors to meld. Stir occasionally.

Stir in the heavy cream and grated Parmesan cheese until fully combined and the sauce is smooth. Return the cooked sausage to the skillet.

Add the fresh spinach to the sauce, stirring until it wilts completely, about 2-3 minutes. Add the cooked rigatoni to the skillet and toss gently to coat the pasta evenly with the creamy sauce. If the sauce seems too thick, add a splash or two of the reserved pasta water until it reaches your desired consistency.

Taste and adjust seasoning with salt and black pepper as needed. Serve immediately, garnished with extra grated Parmesan cheese and fresh chopped parsley, if desired.