Coat a 9-inch cast-iron skillet with cooking spray. Place skillet in a 400° oven.
Combine cornmeal mix and onion; stir well. Combine buttermilk and remaining 3 ingredients; add to dry ingredients, stirring just until moistened.
Remove skillet from oven; pour batter into skillet. Bake at 400° for 25 minutes or until golden.
Cooking Light Low-Fat, Low-Calorie Quick and Easy Cookbook
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