Sweet Onion Cornbread with Cheddar Cheese

3 cups yellow cornmeal
1-1/2 cups flour
1 tablespoon baking powder
1 tablespoon salt
1/2 teaspoon baking soda
4 tablespoons butter, divided
1 quart buttermilk
1 cup chopped Sweet Onion
4 ounces sharp Cheddar cheese, shredded (1 cup)
 
Mix dry ingredients together reserve. Melt 2 tablespoons butter in each of two 9"x9"x2" pans in a 450-degree oven. Meanwhile, stir buttermilk into reserved dry ingredients; fold in onion and cheese. Divide batter between each prepared, hot pan. Bake at 450 degrees until just firm in the center and lightly browned on the edges, about 25 minutes. Remove; let rest 15 minutes before cutting. Cut each pan into 12 pieces. Serve slightly warmed or at room temperature.