Preheat your oven to 400°F and line a large baking sheet with parchment paper.
Carefully slice the spaghetti squash in half lengthwise and scoop out the seeds with a spoon.
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 roasts, heat the olive oil in a large skillet over medium heat.
Add the diced onion and minced garlic to the skillet, sautéing for 3 to 4 minutes until translucent and fragrant.
Add the ground turkey to the skillet, seasoning with sea salt, black pepper, and dried oregano.
Cook the turkey, breaking it up with a spatula, until it is browned and cooked through.
Pour in the marinara sauce and stir in the baby spinach, simmering for 2 to 3 minutes until the spinach has wilted.
Once the squash is done, use a fork to scrape the flesh into long, spaghetti-like strands.
Divide the squash strands into bowls and top generously with the turkey marinara sauce.