Season fish with 1⁄2 teaspoon of salt to taste for about 15 minutes, then lightly dust with 1 teaspoon of cornstarch.
Fry mackerel slices on both sides until just cooked, then allow to cool. Remove bones, flake the flesh and set aside.
Cut peeled potatoes into one-centimetre cubes. Wash off starch with water. Boil potatoes in about 2 cups of water and 1⁄2 teaspoon of salt to taste for about 10 minutes until fluffy but firm. Drain off excess water, then allow the potatoes to cool completely.
Stir together all the bergedil ingredients until combined, being careful to only lightly mash the fish and potatoes. Gather a handful of mashed ingredients, make into a ball then press down into a patty.
Heat oil in a fryer to medium high. Beat the eggs with cornstarch until combined. Dip the bergedil into the batter, then fry until light golden and crispy. Serve immediately as a side dish with sweet chilli sauce or mayonnaise.