Clean each slice of liver to ensure that the "tubes" are trimmed away. Wet the slices with balsamic vinegar, season with salt and a sprinkle of white pepper. Heat the oil in a frying pan over a high heat and fry each rather of bacon until moderately well-done. The fat can stay in the pan to cook the liver. Drain the bacon on kitchen paper and keep warm. saute the onions until golden brown - reduce the flame to medium after a couple of minutes to prevent the onions from burning. You want them to cook gradually to bring out their sweetness. When nice and golden, remove to a plate and keep warm. Finally saute the slices of liver with the garlic cloves, one at a time, until medium rare (with a pinkness still in the centre). Watch this carefully as overcooked liver is dry and useless. When the meat is almost done, discard the garlic cloves and add some onions to the pan, and a few dashes of veal stock. To serve, put the slices of liver with onions on top of some mashed potato and pour the rich gravy around it with the bacon.