In a large bowl, combine the leftover pasta, marinara sauce, olive oil, and Italian seasoning. If using additional veggies or protein, mix them in now. Season with salt and pepper to taste.
Lightly grease an 8x8-inch baking dish with olive oil or cooking spray.
Transfer the pasta mixture into the baking dish, spreading it out evenly.
Sprinkle the shredded mozzarella cheese evenly over the top, followed by the grated Parmesan.
Bake in the preheated oven for 20-25 minutes, or until the cheese is melted, bubbly, and slightly golden.
Remove from the oven and let it cool for 5 minutes before serving.
Scoop into bowls or onto plates and enjoy your comforting leftover pasta bake!