Preheat oven to 350 degrees. Spray a 9 x 13 baking dish with non-stick cooking spray and set aside.
Cooking Spaghetti
Bring a large pot of water to a boil. Stir in uncooked spaghetti and 1/2 TBS salt. Cook spaghetti per directions on back of package for al dente. Drain pasta and place back into pot. Set aside.
Making Red Sauce
In a large skillet over medium high heat break up ground Italian sausage, and add peppers & onions. Cook until sausage is browned up and peppers & onions are softened. Drain excess grease.
Add in pasta sauce, 1/4 of jar filled with water, butter, salt, black pepper, garlic powder, onion powder, and Italian seasoning. Heat & simmer through for a few minutes. Turn off heat and set aside.
Making Alfredo Sauce
In a medium skillet over medium high heat, melt butter and bring to a sizzle. Sprinkle flour over melted butter and whisk together for 1 minute.
Slowly whisk in heavy whipping cream and allow to thicken. Turn down heat and stir in 1 cup parmesan cheese until melted. Season to taste with salt, black pepper, and garlic powder.
Preparing Spaghetti Alfredo in baking dish
Pour alfredo sauce over cooked and drained spaghetti in pot. Pour alfredo covered spaghetti into prepared 9 x 13 baking dish and smooth evenly.
Pour red sauce over alfredo spaghetti and smooth evenly.
Top with 2 cups mozzarella cheese evenly. Sprinkle 1/2 cup parmesan cheese evenly over mozzarella cheese.
Bake in preheated oven for 20 minutes until cheese has melted. Top with parsley. Allow to rest for 5 minutes before serving.