1. Prepare the Spaghetti Squash: Adjust an oven rack to the middle position and preheat your oven to 375°F (190°C). Lightly sprinkle the cut surfaces of the spaghetti squash halves with ¼ teaspoon of coarse salt. Arrange the squash, cut-sides facing down, on a sturdy rimmed baking sheet. Roast for 45-55 minutes, or until the flesh is very tender and easily pierced with a fork. Carefully remove the squash from the oven and allow it to rest until cool enough to handle safely. Using a fork, gently rake the squash flesh lengthwise to create noodle-like strands. Transfer these strands to a colander set over a bowl, cover them to keep warm, and allow any excess moisture to drain while you prepare the rest of the meal.
2. Sauté Aromatics: While the squash is roasting, warm 2 teaspoons of extra-virgin olive oil in a large, heavy-bottomed skillet or Dutch oven over medium heat. Add the finely chopped onions and diced red bell pepper. Sauté, stirring occasionally, for 7-9 minutes, until the vegetables have softened but are not browned. Stir in the minced garlic and chopped fresh oregano, and continue to cook for another minute until their fragrance is released. Transfer half of this aromatic vegetable mixture to a large mixing bowl. Keep the remaining half in the skillet, removing it from the heat temporarily.
3. Form the Turkey Meatballs: To the mixing bowl containing half of the sautéed vegetables, add 3 tablespoons of the grated Parmesan cheese, the whole wheat breadcrumbs, the lean ground turkey, ½ teaspoon of coarse salt, and ½ teaspoon of freshly ground black pepper. Gently combine these ingredients using your hands until just mixed; be careful not to overwork the mixture. Shape the turkey mixture into approximately 20-24 small, uniformly sized meatballs.

Cook Meatballs and Build Sauce:

1. Return the skillet (with the reserved vegetable mixture) to medium heat. Add 1 tablespoon plus 1 teaspoon of extra-virgin olive oil. Once the oil is shimmering, carefully place the turkey meatballs in the skillet in a single layer, ensuring not to overcrowd (brown in batches if necessary). Cook for 5-7 minutes, turning them gently with tongs, until browned on all sides. Transfer the browned meatballs to a clean plate.
2. If the skillet seems dry, add the remaining 2 teaspoons of extra-virgin olive oil. Add the sliced mushrooms, ¼ teaspoon of coarse salt, and ¼ teaspoon of freshly ground black pepper. Cook, stirring occasionally, for 6-9 minutes, until the mushrooms are tender, have released their liquid, and are beginning to caramelize lightly. If the pan bottom becomes too brown, deglaze with a tablespoon or two of water or some of the chicken stock.
3. Ensure the reserved onion-pepper-garlic-oregano mixture is incorporated with the mushrooms. Return the browned turkey meatballs to the skillet. Pour in the low-sodium chicken stock and bring the entire mixture to a gentle simmer. Reduce the heat to low, partially cover the skillet, and let it simmer for 5-7 minutes, or until the meatballs are cooked through and the sauce has slightly thickened.
4. Stir in the fresh baby spinach (a few handfuls at a time if necessary) and cook just until it wilts into the sauce, typically 1-2 minutes.
5. Serve the Dish: To serve, divide the warm spaghetti squash noodles among 8 individual bowls or plates. Ladle a generous portion of the turkey meatballs and mushroom-spinach sauce over the squash. Garnish each serving with a sprinkle of the remaining 3 tablespoons of grated Parmesan cheese. Serve immediately.