1lb / 500g lamb's liver, membrane removed, sliced crosswise into 1/4-in / 6mm slices
2 cups / 475ml milk
½ cup / 125g butter
1 onion, minced
2 tbsp Irish whiskey
½ cup / 120ml heavy cream
1 tbsp wholegrain mustard
1 tbsp chopped fresh chives
Put the liver into a shallow dish large enough to hold it in a single layer; then cover with the milk and sprinkle lightly with salt. Cover and refrigerate overnight.
Rinse the lamb's liver, discarding the milk, and pat dry with paper towels. Melt half the butter in a large skillet over medium heat. Cook the onions, stirring frequently, until soft and beginning to brown, 10 to 12 minutes. Remove the onions from the skillet with a slotted spoon and set aside. Melt the remaining butter in the same skillet and, working in batches, sear the liver over high heat for about 1½ minutes on each side. As the liver is done, transfer to a plate and set aside.
Pour the whiskey into the pan, warm it for about a minute, then carefully ignite it with a kitchen match. When the flames die down, stir in the cream and mustard, scraping up any browned bits on the bottom of the pan. Cook for about 1 minute, then return the onions and liver to the skillet, along with any juices that may have accumulated.
Stir well, season to taste with salt and pepper, and cook for 1 to 2 minutes or until the liver is heated through. Garnish with the chives.