Preheat your oven to 400°F and line a large baking sheet with parchment paper.
Carefully slice the spaghetti squash in half lengthwise and use a spoon to scoop out the seeds and stringy bits.
Drizzle the cut sides of the squash with olive oil and season with a pinch of sea salt and black pepper.
Place the squash halves cut-side down on the baking sheet and roast for 40 to 45 minutes until the skin is easily pierced with a fork.
While the squash is roasting, heat a large non-stick skillet over medium heat and add the ground turkey and diced yellow onion.
Cook the turkey until browned and cooked through, breaking it into small crumbles with a spatula.
Stir in the minced garlic, dried oregano, and the remaining salt and pepper, cooking for 1 minute until fragrant.
Pour the marinara sauce into the skillet and reduce the heat to low, simmering for 10 minutes to allow the flavors to meld.
Once the squash is done, remove it from the oven and use a fork to scrape the flesh into long, spaghetti-like strands.
Transfer the squash strands into the skillet with the turkey marinara, tossing gently to coat everything evenly.
Divide the mixture into bowls and garnish with freshly chopped basil before serving hot.