Wash the mussels, remove the intestines if necessary, and dry them well, otherwise it will splatter during frying.
Heat the clarified butter and fry the salted and peppered mussels lightly golden brown on both sides (not too long, otherwise the mussel meat will be tough!). Keep the clams warm.
Put the onion, garlic and butter in the pan and lightly brown, then add the lemon juice and toss the parsley briefly in it.
Arrange the mussels on the starter plates and pour the sauce over them.