Clean and soak the liver in milk for minimum 1 hour, preferably 3, overnight is arguably better.
Melt 3 tbsp butter on medium heat in your skillet.
Sautee the onions until soft (or cook for 45 minutes and caramelize them. The onions can be long strips, or coarsely diced.
Stir in the sugar and continue to cook until caramelized.
Rinse the liver, clean up any membrane or ikky parts.
Slice liver into 1/2" slices.
Heat 1 tbsp butter on medium heat.
Dredge liver in flour (see below) and brown in butter, about 1 1/2 minutes per side.
Season with salt&pepper, cover with onions.