Slice the mushrooms thinly.Boil the egg in a saucepan for 5 minutes.
Slice the onion into small cubes, chop the garlic finely.
Heat a frying pan, add neutral vegetable oil, heat until it's about to start smoking. Add the mushrooms and stir-fry.Add the onions and garlic to the skillet and cook, stirring, until browned.Pour sherry into the pan, salt and pepper, bring to a boil, reduce heat to low and cook for 10 minutes.
Boil a hard-boiled egg, peel, finely chop or break with your hands.Finely chop the parsley and mix with the egg.
Transfer the mushrooms to a serving dish, sprinkle with the egg and herbs, taste for salt and serve.