Preheat the oven to 400°F. Slice the spaghetti squash in half lengthwise and scoop out the seeds. Drizzle with olive oil and sprinkle with salt, black pepper, and garlic powder. Place cut-side down on a baking sheet and roast for 35-45 minutes, until tender. Let cool slightly, then scrape the flesh into strands with a fork.

In a skillet, sauté the onion in a little olive oil until soft. Add the garlic and cook for 1 minute more. Stir in the marinara sauce, Italian seasoning, and any remaining salt and black pepper. Simmer for a few minutes.

In a large bowl, combine the spaghetti squash strands with the marinara mixture. Stir in the ricotta, half of the mozzarella, Parmesan, and egg, if using. Add the cooked meat, if desired.

Spread the mixture into a greased 9x13-inch baking dish. Top with the remaining mozzarella and a sprinkle of Parmesan. Bake at 375°F for 20-25 minutes, until bubbly and golden on top. Let rest for 10 minutes before serving.