Preheat your oven to 400°F (200°C).
Cut the spaghetti squash in half lengthwise and scoop out the seeds.
Brush the inside of each half with olive oil and season with salt and pepper.
Place the squash halves cut-side down on a baking sheet and roast for 30-40 minutes, or until the flesh is tender and easily shredded with a fork.
In a mixing bowl, combine the marinara sauce, cooked ground turkey or beef, Italian seasoning, and half of the mozzarella cheese.
Once the squash is cooked, carefully flip the halves over and use a fork to shred the flesh into spaghetti-like strands.
Mix the shredded squash with the meat and sauce mixture until well combined.
Spoon the mixture back into the squash halves and top with the remaining mozzarella and Parmesan cheese.
Return the stuffed squash to the oven and bake for an additional 10-15 minutes, or until the cheese is melted and bubbly.
Remove from the oven, let cool for a few minutes, and garnish with fresh basil leaves before serving.