Place the mussels in a large saucepan with the bouquet garni, chopped onions and white wine. Cook them covered over high heat until the mussels open. Take the mussels out of their shells and reserve their juice. Place the mussels on a platter and keep them warm on top of a pan of boiling water.
To make the sauce: make a roux; melt the butter, add the flour and stir until smooth. Strain the mussel juice through a double layer of cheesecloth and whisk into the sauce with the minced garlic, pepper and a little salt. Cook over low heat for 10 minutes, stirring occasionally. Whisk in the crÅme fraöche, boil for a couple of minutes, then off the heat, add the egg yolks one at a time. Coat the mussels with this sauce and sprinkle with parsley. Serve very hot.
