Melt the butter and gently fry the fish for 1 minute. Add all ingredients EXCEPT the wine, egg yolk and cream. Cook for about 10 mins. Add the wine and transfer the fish to a warmed plate. Allow the sauce to bubble for 1 minute then slowly stir in the cream. Reduce heat and mix the egg yolk into the mixture quite briskly. Pour over the fish and serve.
I usually serve with new potatoes and vegetables. As the sauce is quite rich it doesn't work so well with oily fish such as salmon.