Pre-heat oven to 450 degrees. Line baking sheet with foil, (for easy clean up), coat with non-stick cooking spray. Wash peppers, and cut around the stem to remove. Cut peppers in half length-wise, and remove the seeds and ribs inside the peppers. Set peppers on baking pan.
Meanwhile, spray a large frying pan with cooking spray, and turn the heat up to medium high. Add the ground turkey and chopped onion to the pan. Cook until the meat is no longer pink.
Add the beans, corn, salsa and taco seasoning to the skillet (with the turkey) and bring sauce to a boil. Reduce the heat, and simmer, uncovered in pan for 5-6 minutes, or until everything is heated through, then turn off the heat.
Spoon the turkey mixture evenly into the 4 peppers and sprinkle 1 tbs cheese on top of each one. Bake, uncovered, for 10-12 minutes, or until peppers are tender. and remove from the oven.
Top each pepper with 1 tablespoon of sour cream, and tomatoes, (and black olives and lettuce if desired). Serve warm!