Cover the liver with the milk and refrigerate it for 30 minutes, then drain and pat it completely dry.
Cook the bacon in a large skillet over medium heat until crisp, transfer it to a plate and leave 2 tablespoons fat in the pan.
Cook the onions in the bacon fat for 12 minutes until deep golden and soft, then move them to the bacon plate.
Mix the flour, salt and pepper, dredge the liver lightly and shake off the excess.
Melt the butter in the skillet and cook the liver for 2 to 3 minutes per side until browned and 160°F; serve with the onions and bacon.