1. **Cook the sausage:** Heat a skillet over medium heat with a drizzle of olive oil. Sauté chopped onion and minced garlic until the onion becomes translucent. Add the Italian sausage, breaking it into smaller pieces as it cooks, and cook until browned. Remove the sausage mixture from the skillet, strain off most of the grease, and set aside.
2. **Make the Alfredo sauce:** Melt butter in a separate saucepan over medium heat. Sauté chopped garlic in the melted butter until fragrant. Slowly whisk in the heavy cream and bring it to a gentle simmer. Stir in grated Parmesan cheese and whisk until the sauce thickens. Use a teaspoon of flour if necessary for consistency and season with salt, pepper, and optional red pepper flakes.
3. **Prepare the rigatoni pasta:** While the sauce is simmering, cook the rigatoni pasta in a large pot of boiling, salted water until al dente according to package instructions. Drain, reserve a little pasta water, and drizzle with olive oil to prevent sticking.
4. **Combine and bake:** Preheat your oven to 350 degrees Fahrenheit. In a large mixing bowl, combine cooked rigatoni pasta, sausage mixture, and Alfredo sauce, stirring to coat evenly. Transfer the mixture to a greased baking dish. Top with shredded mozzarella cheese and bake for 30 minutes or until bubbly and golden brown on top.
5. **Garnish and serve:** Remove the dish from the oven and let it cool slightly. Garnish with fresh basil for added flavor and color. Serve hot with a side salad or garlic bread for a complete meal.