Heat 1 tbsp oil in a frying pan and add the onions. Keeping the heat quite high toss around to brown to a dark colour around the edges without burning. Add the garlic and toss again.
Pour in the marsala and the vinegar. Bring to a simmer then reduce the heat to its lowest setting so it is barely bubbling. Simmer for 45 minutes then season with salt and pepper.
Meanwhile slice the liver into 3 cm lengths, keeping then very thin. When the sauce is cooked heat the remaining 1 tbsp of the oil and the butter in another frying pan. When the butter foams add the liver slices and sear briefly for 1-2 minutes.
When ready tip on to a warmed serving dish and pour over the hot sauce and onions. SErve immediately.