9: Add 1 egg yolk and put the pan on low heat. Mix in the egg yolk with a whisk...
10: ...and continue beating gently on low heat until the sauce thickens. Important: do not allow the sauce to boil and stop cooking as soon as the sauce thickens. Check the seasoning.
11: Serve on hot plates, one fish fillet per person with the accompaniment of your choice (rice here) and the sauce poured over.
Remarks
For the fish, use whatever is in season (whiting in these photos).