Clear 750 g green beans, cut it lengthwise and boil it in salted water to which you have added 1 small onion and 1 slice of lemon. Drain it and pour in 4-5 tablespoons fried butter. Beat in saucepan over low heat 3 yolks with 1 cup water and 1/2 cup vinegar. When the sauce thickens, pour the beans and stir gently.