Prepare the oven and baking dish: Preheat the oven to 375°F (190°C). Lightly grease a 9-by-13-inch baking dish with the olive oil.

Cook the pasta: Bring a large pot of salted water to a boil. Cook the penne according to the package directions, but stop cooking about 2 minutes before the recommended time. The pasta should be firm enough to hold its shape and still have a noticeable bite. Drain well.

Warm the meatballs and sauce: Place the marinara sauce, minced garlic, Italian seasoning, and optional red pepper flakes in a large skillet or saucepan over medium heat. Stir in the meatballs and simmer for 8–10 minutes, stirring occasionally, until the meatballs are heated through and the sauce is gently bubbling.

Combine the pasta and sauce: Add the drained pasta to the sauce and meatballs. Stir gently until the pasta is evenly coated. Stir in 1/4 cup of the Parmesan, keeping the remaining 1/4 cup Parmesan for the casserole topping.

Assemble the casserole: Transfer the pasta and meatballs to the prepared baking dish. Spread everything into an even layer so the meatballs are distributed throughout rather than concentrated in one area.

Add the cheese: Sprinkle the shredded mozzarella evenly over the top, followed by the remaining 1/4 cup Parmesan. The cheese should cover most of the surface without needing to be pressed down.

Bake covered: Cover the dish tightly with aluminum foil and bake for 20 minutes. Covering the casserole helps the pasta finish cooking without allowing the cheese to brown too quickly.

Finish uncovered: Remove the foil and bake for another 10–15 minutes, until the cheese is completely melted and lightly golden and the sauce is bubbling around the edges. The meatballs should reach at least 165°F (74°C) in the center if checking with a food thermometer.

Rest and serve: Remove the casserole from the oven and let it rest for 10 minutes. Sprinkle with the optional chopped parsley just before serving. Spoon the casserole into portions, making sure each serving gets pasta, sauce, and meatballs.