Place liver in a medium bowl. Add enough milk to cover. Soak for an hour or two. This will take out the bitterness in the liver. Meanwhile, melt 2 tablespoons of the butter in a large skillet over medium heat, and saute the onions until tender.

Remove the onions, and melt the remaining butter in the skillet on low heat. Meanwhile, add the flour to a medium bowl and season to taste with salt and pepper. Remove the liver from the milk, and coat liver slices with the flour mixture.

Turn up heat on the skillet to medium-high and add the liver slices. Cook until brown on one side, turn, and then cook until brown on the other side. Add the onions, reduce heat to medium, and cook a bit longer to taste.