Place the mussels, well washed and with their ‘beards’ removed, in a large deep pan over a low flame. Add no water. The mussels will soon open. Remove them and take the surplus half-shells off them.
Keep the mussels, each now sitting in its remaining half-shell, warm, for example in the top of a large double-boiler. Strain and reserve the mussel juices from the pan.
Melt the butter and stir the flour into it over a low flame. Add the garlic and wine and some of the mussel liquor, enough to produce a fairly thin sauce after 10 minutes’ gentle cooking.
Season the sauce, remove it from the heat , add the egg yolk to bind it, the saffron or turmeric, and the lemon juice. Put the mussels into the sauce and sprinkle a little chopped parsley over all.