Place the sausage in a saucepan and add enough water to cover it.
Add the vinegar, onion, peppercorns and bay leaves along with a little salt.
Bring the contents of the pan to the boil. As soon as it starts to bubble gently, turn the heat down so it only simmers with small bubbles. Leave it like this for 15 minutes.
Turn off the heat and leave the sausage to rest in the water for about 10 minutes.
Meanwhile, melt the butter with the oil in a frying pan and let it foam up well before adding the sausage.
Fry the sausage over a medium heat for about 8-10 minutes on each side - take care that the pan is not too hot, or the sausage will burst.You can prick the sausage with a thin metal skewer, which helps stop it bursting.
Serve the pan-fried sausage with the accompaniments of your choice.