Preheat the oven to 350°F (175°C) and prepare an 8×8-inch baking dish by spraying it with cooking spray.
Cook the rigatoni according to package directions in a large pot of salted water until al dente. Drain and drizzle some olive oil over it to prevent sticking. Set aside.
In a large skillet over medium-high heat, sauté the chopped onion and minced garlic in olive oil until translucent, about 2-3 minutes.
Add the sweet Italian sausage and cook thoroughly. Remove from heat and stir in basil, salt, and pepper. Transfer to a large bowl and set aside.
For the Roasted Garlic Alfredo Sauce: Remove roasted garlic cloves from skins and mash. In a medium saucepan, melt butter over low heat. Add heavy cream and mashed garlic, warming without boiling. Mix in Parmesan cheese, salt, and pepper, whisking until melted.
In a small bowl, combine flour and heavy cream until smooth. Add this to the cheese sauce, stirring until thickened.
Pour the Alfredo sauce over the sausage mixture and stir to combine. Add rigatoni and mix well. Transfer to the prepared baking dish and sprinkle shaved Parmesan on top.
Bake for 30 minutes until bubbly and lightly browned.
Serve hot and enjoy!