You can use a piece of back or streaky bacon for this – it's up to you – and choose between smoked or unsmoked; I prefer the smoked, personally. You can also add some peeled, small potatoes into the pan along with the bacon.
A piece of streaky or back bacon weighing about a kilo
2 onions, halved
15 black peppercorns
1ltr dry cider
1-1.5ltrs beef or chicken stock
1 bay leaf
About a tablespoon of cornflour
Place the bacon in a large saucepan with the onions, peppercorns, cider, stock and bay leaf. Bring to the boil and simmer gently for about 1½ hours or until the bacon is tender.
Remove about a third of the liquid and transfer to a saucepan. Boil it rapidly until you end up with about 300-400ml.
Dilute the cornflour in a little cold water and stir enough into the reduced stock just to thicken it slightly, then whisk in the butter and season to taste if necessary.
To serve, remove the bacon from the stock, cut into 1cm-thick slices, arrange on a serving dish and spoon over the sauce.