Preheat the oven to 400°F (204°C). Grease a 15x10x1-inch rimmed baking sheet with cooking spray.
Cook the 16 oz of spaghetti in a large pot according to package directions until al dente. Drain in a colander and rinse briefly with cool water to stop cooking; set aside.
In a large mixing bowl, whisk together 2/3 cup milk, 2 large eggs, 1/2 tsp Italian seasoning, 1/2 tsp garlic salt, and salt and pepper to taste until blended.
Add the cooked spaghetti to the egg-milk mixture, then stir in 1/2 cup grated Parmesan and 1 cup of the shredded mozzarella until evenly combined.
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 (177°C).
Top the baked spaghetti with the 24 oz jar of spaghetti sauce, sprinkle the remaining 2 cups shredded mozzarella over the sauce, and add your desired pizza toppings.
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 from the oven, let rest a few minutes, then slice and serve.