01 - Boil the potatoes in salted water for 12 to 15 minutes until tender. Drain thoroughly and mash until smooth, then allow to cool slightly.
02 - Simmer fish fillets in hot water for 5 to 6 minutes until just cooked through. Drain and flake into large pieces.
03 - In a mixing bowl, gently fold mashed potatoes, flaked fish, spring onions, parsley, Dijon mustard, and half of the beaten egg. Season with salt and pepper.
04 - Form the mixture into 8 equal patties and refrigerate for 15 minutes to firm up.
05 - Arrange three shallow dishes: one with flour, one with the remaining beaten egg, and one with breadcrumbs.
06 - Dredge each patty first in flour, then dip into the beaten egg, and finally coat with breadcrumbs.
07 - Heat olive oil over medium heat in a frying pan. Fry fishcakes for 3 to 4 minutes on each side until golden brown and crispy. Drain on paper towels.
08 - Whisk together mayonnaise, minced garlic, lemon zest, lemon juice, Dijon mustard, salt, and pepper until smooth.
09 - Serve fishcakes hot accompanied by the lemon mayonnaise on the side.