Preheat the oven to 400°F. Cut the spaghetti squash in half lengthwise and scoop out the seeds. Place the halves cut-side down on a baking sheet and roast for 35-40 minutes until the flesh shreds easily into strands with a fork.

While the squash roasts, combine the ground turkey, egg, breadcrumbs, half the minced garlic, half the chopped basil, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a bowl. Mix gently and form into 16 meatballs.

Heat 1 tablespoon olive oil in a large skillet over medium heat. Add the meatballs and cook, turning occasionally, until browned on all sides and cooked through, about 10 minutes. Remove the meatballs to a plate.

In the same skillet, add the remaining tablespoon of olive oil along with the onion. Cook for 3 minutes until softened. Stir in the remaining garlic and cook 30 seconds more.

Add the diced tomatoes, remaining 1/2 teaspoon salt, and 1/4 teaspoon pepper to the skillet. Simmer for 12-15 minutes until the tomatoes break down into a chunky sauce.

Return the meatballs to the skillet and simmer 3 minutes to coat them in sauce. Scrape the roasted spaghetti squash strands from the skin and divide among four plates. Spoon the meatballs and sauce over the squash and finish with the remaining fresh basil.