01 - Boil potatoes in salted water for 12 to 15 minutes until tender. Drain and mash until smooth. Allow to cool slightly.
02 - Poach fish fillets in simmering water for 5 to 6 minutes until just cooked through. Drain and flake into large pieces.
03 - In a bowl, gently mix mashed potatoes, flaked fish, spring onions, parsley, Dijon mustard, and half the beaten egg. Season with salt and pepper.
04 - Form mixture into 8 equal patties. Refrigerate for 15 minutes to firm.
05 - Arrange three shallow dishes containing flour, remaining beaten egg, and breadcrumbs. Coat each patty sequentially in flour, egg, and breadcrumbs.
06 - Heat olive oil in a frying pan over medium heat. Fry fishcakes for 3 to 4 minutes per side until golden and crisp. Drain on paper towels.
07 - Whisk mayonnaise, minced garlic, lemon zest and juice, Dijon mustard, salt, and pepper until combined.
08 - Serve fishcakes hot accompanied by lemon mayonnaise.