Preheat oven to 400°F. Carefully cut the spaghetti squash in half lengthwise and scoop out the seeds. Drizzle with olive oil and place cut-side down on a baking sheet. Roast for 35–40 minutes or until tender. Let cool slightly, then use a fork to scrape out the strands.

In a large skillet over medium heat, sauté the diced onion until softened, about 5 minutes. Add garlic and cook for another minute. Add ground turkey and cook until browned and cooked through. Season with salt, pepper, and Italian seasoning.

Stir in the marinara sauce and bring to a simmer. Let cook for 5 minutes, then remove from heat.

In a large mixing bowl, combine the roasted spaghetti squash strands with the turkey marinara mixture. Mix well and transfer to a greased 9x13" baking dish. Top evenly with mozzarella and parmesan.

Reduce oven temperature to 375°F. Bake the casserole uncovered for 20–25 minutes, or until the cheese is melted and bubbly. Broil for 1–2 minutes at the end if desired for a golden top.

Let cool slightly before serving. Garnish with fresh herbs, if desired.