Preheat oven to 425 degrees. Coat a 9 x 9” baking pan with cooking spray.
In a microwave-safe bowl, add onions, red bell peppers, and butter. Cook in microwave on high for about 3-4 minutes until soft. Set in refrigerator to chill for 10 minutes.
Mix together the cooled onion mixture, ham, milk, eggs, egg whites, garlic powder, and pepper in a large bowl. Blend well. Stir in cornbread mix and only stir until blended. The batter will be a bit bumpy. Spread cornbread batter into prepared pan.
Using a rubber spatula, spread yogurt over the top of the casserole. It won’t cover completely.
Sprinkle cheese evenly over top of yogurt.
Bake for 45 minutes so the entire casserole will be cooked through. It will look very dark golden brown.
Let stand for 10 minutes before cutting into 6 slices. This casserole may be made ahead of time, refrigerated, and reheated. It also freezes great!