Lambs’ liver with onions

Heat 70 g of the butter and the oil in a frying pan and cook the onions slowly over a low heat.
Cook for about 30 minutes, stirring often until the sauce becomes sweet and sticky.
Remove from the heat and keep warm.
Melt the remaining butter in the same pan.
When sizzling, add the liver.
Brown it for a minute or two, then cook the other side for another minute.
The quicker the liver is cooked, the more tender and moist it will be – it should be pink inside.
Remove from the pan and keep warm.
Add the vinegar to the pan and scrape all the pan juices together, adding salt and freshly ground black pepper to taste.
Add the crème fraîche and allow to melt.
Pour over the liver and serve with the onions and mashed potato.