Preheat oven to 425°. Place a well-seasoned 8" cast-iron skillet in hot oven 10 minutes. Add 1/4 cup butter to skillet, and heat 5 additional minutes.
Combine remaining 1/4 cup butter, cornmeal mix, and next 4 ingredients, stirring well with a wire whisk; let stand 2 minutes.
Arrange onion and sage leaves in bottom of hot skillet; pour batter slowly over onion and sage.
Bake at 425° for 35 to 40 minutes or until a wooden pick inserted in center comes out clean. Invert skillet over serving plate to remove bread. Cut bread into wedges to serve.
*You can substitute 1 cup chopped onion, chopped green pepper, and chopped sweet red pepper mixed together for the frozen seasoning blend.