Smoked trout fishcakes with walnut sauce

I love a fishcake, a stalwart piece of comfort food, but they are often too complicated, with too many ingredients. I believe a fishcake should be simply about fish and potato. The ratio of fish to potato here is perfect, thanks to some advice from Arnie, a kind cook with a café in Whitby who, despite his squashed nose and granite stature, has a sensitive touch in the kitchen. His is the best fishcake I have ever eaten!

Do use smoked haddock if you prefer. Being col