If you want to tone down the strong flavor I would suggest soaking pieces in a bowl of milk for an hour before beginning the next steps. This is optional but suggested. Pat outsides dry if you do, then continue.
In a bowl, mix all-purpose flour with salt and pepper. Rinse and dry the liver with paper towels on the outside. Roll each piece in flour on all sides.
Heat the frying oil and fry the chicken liver on low heat for 5 minutes on one side. Turn the liver over to the other side, cover the pan with a lid, and fry for another 4 minutes on low heat. Then transfer to a plate.
Fry the chopped onion in butter for 5-6 minutes on low heat, stirring occasionally. Add sugar and stir, fry for another 3-4 minutes. Lay the onion on top of the liver and serve.