Preheat oven to 350 degrees. Heat oil in a large skillet on medium heat. Add onion, garlic, and bell pepper. Saute until onions are soft, about 3-4 minutes. Add ground turkey and cook until lightly browned, breaking up meat with spatula. Add taco seasoning, corn, beans and tomato sauce. Stir well to combine. Cook until heated through. Transfer to a casserole dish. Sprinkle cheese on top. Bake until cheese is melted and casserole is bubbly, about 20-25 minutes.