HEAT oven to 400°F Heat oil over medium heat in 10 1/2-inch cast iron skillet. Add 1 cup onion, garlic, green pepper, cumin and chili powder; sauté about 3 to 5 minutes or until vegetables are tender. Add remaining filling ingredients; simmer about 10 minutes.
BEAT egg in medium bowl. Add milk, butter and cornbread mix; mix well. Stir in 1/4 cup onion and cheese. Pour over chicken chili in skillet.
BAKE 25 to 30 minutes or until cornbread is golden brown. Top with sour cream, salsa, and/or cilantro, if desired.