Peel onions and cut them into not too thin rings. Heat 2 tablespoons of oil in a wide saucepan. Fry the onion rings over medium heat while turning. Season with salt and pepper.
Stir in honey, port wine and vinegar. Bring to the boil and simmer at medium heat for about 10 minutes, stirring frequently. In the meantime, rinse liver briefly in cold water and pat dry. Wash sage, shake dry.
Pluck the leaves from the stems. Put the flour on a flat plate. Heat 2 tablespoons of oil in a coated pan. Season liver with pepper. Turn in the flour and tap off excess flour.
Then fry in hot oil at medium to high heat for about 2 minutes on each side. When turning, add butter and sage leaves to the pan. Season the liver with salt only after frying, otherwise it will become hard and dry.
Remove liver and sage from the pan and keep warm for a short time. Dissolve frying fat with 1-2 tablespoons of water, pour into the onions. Bring to the boil briefly and season to taste. Arrange liver and port wine onions. Serve with baguette or mashed potatoes.