Discard any broken mussels and those that do not close when sharply tapped. Scrub well and pull out the little beards.
Put the white wine, onion and garlic into a heavy lidded frying pan and brig to the boil. Boil for a minute then add the mussels, cover tightly and cook for a minute.
Shake the pan and remove any mussels that have opened, then repeat the process, keeping the opened mussels in a covered bowl. Mussels only need to be cooked lightly so they are plump and still full of their own juices.
Discard any mussels that remain closed then strain the cooking broth into a jug and set aside.
Melt the butter in a small saucepan, scatter on the flour and cook, stirring, for about 3 minutes. Gradually pour in the mussel broth, stirring constantly, then slowly pour in the milk, stirring.
Beat the egg yolk, curry powder, saffron, turmeric and cream together. Add to the sauce, stirring well, but don't let it boil.
Remove the top shells of about half the mussels and discard. Pile all the mussels into 4 warmed shallow bowls and pour the sauce over the top to serve.
I like to accompany with some oven chips or crusty bread.