In A Medium Skillet, Melt Butter Over Medium Heat. Stir In Onion And Saute Until Softened. Stir In Sugar And Continue To Cook Until Onion Caramelizes. Rinse Liver And Remove Thin Outer Membrane. Slice Liver Into 1/4 To 1/2 Inch Thick Slices. Heat Remaining Butter In A Medium Skillet Over Medium Heat Dredge Liver Slices In Flour And Brown In Melted Butter For About 1 To 1 1/2 Minutes Per Side. Remove From Heat And Season To Taste With Salt And Pepper. Serve Liver Smothered With Caramelized Onions.