Peel and roughly cut the potatoes into large chunks. Cook for 10 to 15 minutes, until completely tender.
In the meantime: drain the tinned fish. Flake, season with the pepper, salt and cayenne (if using) and set aside in a large bowl.
Drain the potatoes and mash them to your desired consistency in the pan. I like mine with a remnant of chunkiness.
Add the potatoes to the fish mixture and mix together, again: to your desired consistency. I like there to be chunks of fish and potato left, so it's not so much of a coherent mush but more of a textured affair. I'm not judging you coherent mush people however.
Melt some butter in a hot skillet or non-stick frying pan.
Form your potato-fish mixture into four patties, lightly dust them and fry until golden in the butter.
I say this is a recipe for 2, but I tend to eat this by my lonesome with a nice lightly dressed green salad and the below sauce if I happen to be sitting on some cream. Mayo works fine too.