Clean the mushrooms. Dice the onions and garlic and cut the leek into fine strips.
First sweat the onions in olive oil until they are translucent (do not let them get any color), then add the garlic, leek and mushrooms and fry everything gently for about 5 minutes. Add the sherry, bring to the boil and season with salt and pepper.
Chop the parsley and sprinkle over it before serving.