Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9×13 inch baking dish.
Step 2: In a large bowl, combine the leftover spaghetti with 1.5 cups of marinara sauce. Toss gently to ensure the pasta is evenly coated. If your leftover spaghetti is very dry, add a little more sauce until lightly coated.
Step 3: In a separate medium bowl, mix together the ricotta cheese, beaten egg, 1/2 cup of the shredded Parmesan cheese, chopped fresh parsley, salt, and black pepper. Stir until well combined.
Step 4: Spread half of the sauced spaghetti mixture evenly into the bottom of your prepared baking dish. Press it down lightly to form a compact layer.
Step 5: Carefully spread the entire ricotta cheese mixture over the first layer of spaghetti, using the back of a spoon or spatula to create an even layer.
Step 6: Top the ricotta layer with 1 cup of the shredded mozzarella cheese.
Step 7: Spread the remaining half of the sauced spaghetti over the mozzarella layer, pressing down gently.
Step 8: Cover the top with the remaining 1 cup of shredded mozzarella cheese and the remaining 1/4 cup of Parmesan cheese.
Step 9: Cover the baking dish loosely with aluminum foil.
Step 10: Bake for 25 minutes. Then, remove the foil and bake for another 10-15 minutes, or until the cheese is bubbly and golden brown on top.
Step 11: Once out of the oven, let the casserole rest for 5-10 minutes before serving. This allows the layers to set, making it easier to cut and serve.