Cook the spaghetti squash: Preheat the oven to 425 degrees. Cut the squash in half lengthwise and discard the seeds and membrane. Drizzle with a splash of olive oil and sprinkle with a pinch of salt and pepper. Place, cut side down, on a baking sheet. Bake in the preheated oven for 35-45 minutes, depending on the size of the squash, until fork tender on the inside. Scrape out the spaghetti-like insides of half of a large or the entire whole small squash and set aside.
Adjust the oven temperature to 400 degrees Fahrenheit. Heat a large oven safe skillet over medium heat and add the oil. Once the oil is heated, add the diced onion and bell pepper. Saute until mostly tender, stirring occasionally, about 5 minutes.
Stir in the ground turkey and cook, stirring and breaking up the meat, until mostly cooked, about 5-7 minutes. Stir in the taco seasoning, black beans, and corn. Cook for another 3-5 minutes.
Stir in the spaghetti squash until well combined. Taste and add a pinch of salt and pepper if needed. Spread the mixture in a single even layer and sprinkle evenly with the shredded cheese.
Bake in the preheated oven for about 15 minutes or until the sides are bubbly and the cheese is melted.
Serve hot with chopped cilantro and sour cream if desired.