Preheat your oven to 400°F. Cut the spaghetti squash in half lengthwise and scoop out the seeds. Drizzle the cut sides with olive oil and season with salt and pepper. Place the squash cut-side down on a baking sheet and roast for 40-45 minutes, or until tender and easy to shred with a fork.
While the squash is baking, heat a drizzle of oil in a large skillet over medium-high heat. Add the ground turkey and cook for 5-7 minutes, breaking it apart as it browns.
Add the diced onion, Italian seasoning, and garlic. Sauté for a few minutes, or until the onion begins to soften.
Pour in the marinara sauce and stir well. Reduce the heat to low and let the mixture simmer for 5-10 minutes so the flavors can come together. Taste and season with salt and pepper as needed.
To make the breadcrumb topping, mix the breadcrumbs, melted butter, and Italian seasoning in a small bowl until evenly combined.
Once the spaghetti squash is tender, use a fork to gently fluff the strands while keeping them inside the shells. Spoon the turkey marinara mixture into each half, then top with shredded cheese and the seasoned breadcrumbs.
Return the stuffed squash to the oven for about 5 minutes, or until the cheese has melted and the breadcrumbs are lightly golden. Serve warm and enjoy!