Put the cod fillets in a pan and cover with water. Bring to a simmer and cook for 3 minutes until the fish is opaque. Drain and flake with a fork.
Put the mashed potato in a large bowl and mix in the flaked cod, half the spring onions, 1 tbsp flour, the chopped prawns and half the tartare sauce. Season with freshly ground black pepper and shape into 8 even-sized fishcakes.
Put the remaining flour, egg and breadcrumbs in 3 separate dishes. Lightly dust each fishcake with flour, then dip into the beaten egg and coat in breadcrumbs.
Heat the oil in a large frying pan and fry the fishcakes for 5 minutes, turning once, until golden. Mix the remaining spring onions with the salad and serve with the fishcakes and the remaining tartare sauce.