Preheat the oven to 400°F (200°C) and grease a 15x10x1-inch rimmed baking sheet with cooking spray.
Cook the spaghetti in a large pot according to package directions until al dente; drain and rinse with cool water, then set aside.
In a large mixing bowl, whisk together milk, eggs, Italian seasoning, garlic salt, and salt and pepper to taste until combined.
Add the cooked spaghetti, grated Parmesan, and 1 cup of the shredded mozzarella to the egg mixture and toss until evenly coated.
Spread the spaghetti mixture evenly in the prepared baking sheet and bake at 400°F for 15 minutes.
Remove the pan from the oven and reduce the oven temperature to 350°F (175°C).
Top the baked spaghetti layer with the jar of spaghetti sauce, sprinkle the remaining 2 cups of shredded mozzarella over the sauce, and add your chosen pizza toppings evenly across the surface.
Return the pan to the oven and bake at 350°F for 20 to 25 minutes, or until the cheese is melted and bubbly; remove and let rest a few minutes before slicing and serving.
