In a large bowl add the cooked potatoes and season with a little salt and pepper. Add the butter and then mash them up using a potato masher. Add in the dill and the tinned mackerel plus the egg. Using a wooden spoon, stir everything together until completely combined.
Cover the mixture then refrigerate until the mixture becomes former and less sloppy in consistency. This shouldn't take too long, maybe 20-30 mins at most.
Heat up a skillet or non stick frying pan and pour in a little oil. Whilst the oil heats up, take the fish cake mixture and form into patties, by first wetting your hands so it doesn't stick then rolling into balls and flattening down.
Add the fish cakes into the hot oil and fry in batches over a medium heat until warmed through and golden on both sides. Leave on some kitchen towel to remove any excess oil then serve up and enjoy. :)
