Sausages punctured with a fork and put in a deep frying pan, by pouring water 1 finger. Cover the pan and is heated until it begins to boil slightly. Occasionally turn the sausages until the water boil off. Then add the oil and fry over medium heat, uncovered, until they become golden brown. Chop crescents and add to the pan. Fry until the onions start becomes golden, then add the beer. Leave on low heat until the beer is evaporated. Serve comes mustard.