Preheat the oven to 400°F (200°C).
Cut the spaghetti squash in half lengthwise and scoop out the seeds. Place the halves cut-side down on a baking sheet. Bake for 30-35 minutes, or until tender.
Meanwhile, in a large skillet over medium heat, add the olive oil. Once hot, add the diced onion and cook until translucent, about 3-4 minutes.
Add the minced garlic and diced bell pepper to the skillet and sauté for another 2-3 minutes.
Stir in the ground turkey (or beef) and cook until browned and fully cooked, about 7-10 minutes. Drain excess fat if necessary.
Mix in the taco seasoning, black beans, corn, and salsa. Stir well and let simmer for about 5 minutes.
Once the spaghetti squash is done baking, use a fork to scrape the flesh into noodle-like strands.
To serve, place a portion of spaghetti squash in a bowl, and top with the taco mixture. Sprinkle cheese on top and let it melt.
Garnish with fresh cilantro and any desired toppings before serving.