In a large pot of boiling salted water, add the rigatoni and cook according to package instructions until al dente. Drain and set aside.
In a large skillet over medium heat, add the olive oil. Once hot, crumble the Italian sausage into the skillet, breaking it apart with a spoon. Cook until browned and cooked through, about 5-7 minutes.
To the skillet with the sausage, add the chopped onion and minced garlic. Sauté for an additional 2-3 minutes until the onion becomes translucent and fragrant.
Stir in the fresh spinach until wilted. Add the can of diced tomatoes (with juices), red pepper flakes, oregano, salt, and black pepper. Stir well to combine all ingredients. Allow to simmer for 5 minutes, letting the flavors meld.
Add the cooked rigatoni to the skillet, tossing it with the sausage and spinach mixture until evenly coated. If the sauce is too thick, add a splash of water or chicken broth to reach desired consistency.
Stir in the grated Parmesan cheese until melted and creamy. Taste and adjust seasoning as necessary.
Divide the sausage rigatoni among plates and garnish with additional Parmesan cheese and fresh basil leaves if desired.
Notes
Adjust red pepper flakes to taste for desired spiciness.