Cook rigatoni in a large pot of salted water to desired tenderness. Reserve about 1 cup of the water to make the sauce and drain the remaining water from the pasta.
Meanwhile, begin browning sausage in a large skillet over medium heat. When sausage is about halfway cooked (some pink, some brown sausage in pan), add the diced onion and continue cooking until the sausage is cooked and the onion is soft.
Add butter, garlic, Italian seasoning, and sundried tomatoes to the skillet and cook over medium heat 2-3 minutes longer.
Add the cream and reserved pasta water to the sausage mixture and bring to a simmer. Simmer 3-4 minutes.
Stir the spinach into the sauce (it will wilt quickly and does not need to be cooked further), followed by the pasta.
Simmer 1-2 minutes longer to thicken the sauce as needed.