Preheat oven to 350°F (180°C). Spray a medium, nonstick skillet with nonstick cooking spray and place over medium-low heat. Add onion, garlic, chili powder, cumin, salt and pepper. Cook, partially covered and stirring often, until very soft, about 7 min. Increase heat to medium. Crumble in turkey and brown well. Drain off any fat. Add tomatoes and simmer until juices evaporate, 7 to 10 min.
Meanwhile, blanch peppers in a large pot of boiling water for 2 min. Drain well. Place cut-side up on a rimmed baking sheet.
Combine hot meat mixture with rice and corn. Stir in egg and cilantro. Divide mixture evenly between pepper halves, mounding as needed. Bake until peppers are softened, 10 to 15 min. Garnish with fat-free sour cream. Serve with spinach tossed with dressing.