How to Make It
Preheat oven to 425°. Combine cornmeal, flour, sugar, baking powder, baking soda, and salt in a bowl. Combine buttermilk, 3 tablespoons melted butter, and egg in a bowl. Stir roasted red bell peppers and onion into buttermilk mixture. Stir buttermilk mixture into cornmeal mixture.
Place 1 tablespoon butter in a 10-inch cast-iron skillet; place skillet in oven 1 minute. Remove pan from oven; pour batter into pan. Bake at 425° for 25 to 28 minutes.