Season the slices of liver and coat them in plain flour. Heat 2 tbsps of the oil in a heavy based frying pan and cook the liver for about 1 1/2 minutes on each side until lightly coloured. Remove from the pan and set aside to rest.
Tip the old oil out of the pan and add 1 tablespoon fresh oil. Add the finely sliced bacon strips and fry over a medium heat for 2-3 minutes until the fat starts to run.
Add the sliced onion and cook over a high heat for 5-6 minutes until soft. Put the liver back into the pan and toss everything together.
Add the sage and the sherry vinegar, check the seasoning and serve immediately.