Brush the mussels well under cold running water and remove any "beards". Throw away open mussels
Peel and finely dice the onions and garlic. Heat the oil in a large pot. Fry the onions and garlic until transparent. Take out half and put aside. Deglaze the rest with wine and 1/2 l water. Boil up and season. Add mussels, bring to the boil again and cook covered for 8-10 minutes. Shake pot in between
Wash the parsley and chop it, possibly except for something to garnish. Lift out the mussels. Remove the mussel flesh, throw away closed mussels
Fill the mussel meat, parsley and onions into 4 greased ovenproof moulds or 1 large mould. Spread crème fraîche over them and sprinkle with coarse pepper. Gratinate in the preheated oven on the highest setting or under the grill for 3-5 minutes. Garnish if necessary