Preheat your oven to 375°. Cut each squash in half lengthwise. Scoop out the seeds and discard. Lightly drizzle with extra virgin olive oil over each squash and season with salt and pepper.
Place squash halves, cut side down, on a parchment paper lined baking sheet. Bake for about 45 minutes.
While the spaghetti squash is in the oven: prepare the filling. Using a large pan or skillet, heat 1 tablespoon of olive oil over medium high heat. Add the ground turkey and cook until all browned and cooked through. Add the onions and zucchini and sauté for about 3 to 4 minutes. Season with salt and pepper.
Add in the Italian seasoning, garlic powder and fennel seeds. Mix to combine. Add in the spinach and stir to combine. Let cook for 2 to 3 minutes. Pour in the marinara sauce and stir to combine and let cook for a few minutes to warm through.
Remove the spaghetti squash from the oven and turn oven temp to broil. Let stand about 2 to 3 minutes to cool a little. When ready, using a fork, carefully scrape the inside of the squash to create spaghetti-like strands. Scoop the turkey filling and spoon on top of each squash. Sprinkle on the shredded Mozzarella cheese (if using) and place back into the oven (broil setting) for just 2 to 3 minutes, or until cheese has melted and is slightly browned. Remove from the oven, serve and enjoy while hot!