Calves’ Liver with Sweet and Sour Onions

• 8 thin slices calves’ liver, 100 g/3½ oz each

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