To make the sweet and sour onions, drop the onions in boiling water to make it easier to peel them. Place the peeled onions in fresh boiling water and boil for 5 minutes. Drain. Melt the butter in a saucepan, add the sugar and stir over a medium heat for a few minutes. Add the wine vinegar and continue to stir. Add the onions and stir well. Cover the pan and place in the oven at 160°C/325°F/gas 3 for about 10 minutes, or until golden.
Season the flour with salt and pepper and dust the liver. Heat
Pour this sauce over the liver and garnish with the onions and sage.