Preheat the oven to 425 degrees F.
For the Alfredo sauce: Add all the ingredients into a medium saucepan except the Parmesan cheese. Cook over medium high heat until the sauce starts to thicken. Add the Parmesan and stir and continue cooking until is incorporated and the cheese has completely melted in to the sauce.
Add the cooked pasta to a 9-by-13-inch baking dish, drizzle the pesto over it. Next pour over the Alfredo sauce and toss to coat. Top with cooked Italian sausage. Sprinkle some cooked onion next followed by the cooked red pepper. Sprinkle with the Italian seasoning. Then pour the marinara sauce on top and sprinkle with a little red pepper flakes.
Cover with the shredded mozzarella and bake until melted, about 15-20 minutes. Remove and let sit for a few minutes. Garnish with the chopped parsley and freshly grated Parmesan. Enjoy!
