Heat the oven to 190C/170C fan/gas 5. Put the fish in a roasting tin, drizzle over some oil and cook for 10-15 mins until cooked through. Allow to cool, then flake into chunks, removing any skin and bones.
For the pesto, blitz together all the ingredients with a pinch each of salt and pepper, adding 1 tbsp water if it looks too thick.
Mix the fish, mashed potato, pesto, spring onion and egg with a little more seasoning, being careful not to break up the fish too much. Mix in 1-2 tbsp breadcrumbs if the mixture feels too wet. Shape into 6-8 cakes, put on a tray and chill in the fridge for 1 hr.
For the crumb, prepare a bowl of seasoned flour, a bowl of beaten egg and a bowl of breadcrumbs. Coat the fishcakes in the flour, then the egg, then breadcrumbs. Set aside. Can be frozen for up to one month. Defrost thoroughly before cooking.
Heat the 2 tbsp vegetable oil in a frying pan and fry the fishcakes (you may have to do it in batches) until golden brown, about 3-4 mins on each side. Serve with salad, tartare sauce and lemon wedges.