Bring a large pot of salted water to a boil. Add the rigatoni and cook according to the package instructions until al dente. Reserve 1 cup of pasta water before draining.
Drain the pasta and set aside.
2. Prepare the Sausage Mixture
Heat olive oil in a large skillet over medium heat. Add the garlic sausage and cook until browned and fully cooked, about 5-7 minutes. Remove from the skillet and set aside.
In the same skillet, add the chopped onion and cook for 2-3 minutes until softened.
Add the minced garlic and red pepper flakes (if using), cooking for another minute until fragrant.
