Preheat the oven to 200°C, fan 180°C, gas mark 6. Place the fish on a lightly greased baking tray. Bake for 10-12 minutes, or until just cooked through.
Flake the fillets, removing any bones, then leave to cool slightly.
Meanwhile, place the potatoes in a large pan of water and bring to the boil. Cook for 10-15 minutes until tender, then drain well and mash. Leave to cool slightly.
In a bowl, mix together the mash, onion, lemon zest and parsley. Season well. Stir in the salmon, without breaking it up too much so there are still chunks visible.
Shape the mixture into 8 rounds. Place on a lined baking sheet and chill in the freezer for 10 minutes.
Lightly coat the fishcakes in the flour, shaking off the excess. Dip the flour-coated cakes in the beaten egg, then the breadcrumbs to form a complete coating.
Heat the oil in a deep pan. Fry the fishcakes in batches for 2-3 minutes until golden, then transfer to a baking tray using a slotted spoon. Bake in the oven for 10-15 minutes, until piping hot.
Mix the dill into the mayo and serve with the fishcakes.
Cook's tip: Frying the fishcakes before baking ensures you get a crispy breadcrumbed coating.