In a heavy skillet cook the bacon over moderate heat until it is crisp, transfer it to paper towels to drain, and crumble it. Pour off all but 1 tablespoon of the fat from the skillet and in the fat remaining in the skillet cook the onion over moderately low heat, stirring, until it is softened. Into a bowl sift together the cornmeal, the flour, the baking powder, the baking soda, and the salt. In another bowl whisk together the eggs, the butter, the sour cream, and the milk, stir in the bacon, the onion, and the cornmeal mixture, and beat the batter well. Divide the batter among 12 well-buttered 1/3-cup muffin tins and bake the muffins in the middle of a preheated 425°F. oven for 20 minutes, or until they are golden. Turn the muffins out onto a rack and let them cool.