Melt the butter and add the potato, cheese and seasoning. Brown the breadcrumbs and sprinkle around the inside of a well-greased ring mould. Line it with the potato mixture, reserving enough to cover the top. Flake the fish and mix with the sauce and the parsley, egg yolk and grated lemon.
Place in the lined mould, cover with the rest of the potato and bake in a moderate oven