Bring a pot of salted water to a boil and cook the rigatoni according to the package directions, usually about 10-12 minutes, until al dente. Drain and set aside, reserving about a cup of pasta water.

In a large skillet, heat a drizzle of olive oil over medium-high heat. Crumble in the Italian sausage and cook until it's golden and cooked through, about 5-7 minutes. Stir occasionally.

Add the minced garlic and Italian seasoning, cooking until fragrant—about a minute.

Pour in the chicken or vegetable broth and heavy cream, letting the mixture come to a gentle simmer. Allow it to thicken slightly while stirring, about 3-4 minutes.

Add the cooked rigatoni, tossing it gently to combine with the sauce. If it looks too thick, gradually add some reserved pasta water to achieve desired consistency.

Fold in the spinach until it just wilts, then season with salt, pepper, and optional red pepper flakes.

Finish with a sprinkle of grated Parmesan cheese, allowing it to melt into the sauce, and serve hot.