Cook the potatoes in a saucepan of boiling water for 15 minutes, or until tender.
Meanwhile, put the fish in a deep non-stick frying pan with the milk and seasoning. Bring almost to boiling point, then reduce the heat, cover and poach for 5 minutes, or until it flakes when tested with a fork. Remove the fish and flake the flesh, discarding any bones. Reserve the milk.
Drain the potatoes and mash with a fork. Add the fish to the potatoes with the spring onions, lemon zest, pesto and 1½ tablespoons of the poaching milk to make a soft mixture. Season to taste. Allow the mixture to cool.
Shape the fish mixture into eight thick flat cakes and dust all over with flour. Combine the breadcrumbs and parmesan on a plate. Coat the fishcakes in the beaten egg and then the crumbs. Chill until ready to cook.
Preheat the oven to 190˚C. Place the fishcakes on a non-stick baking tray with the halved tomatoes. Bake for 20 minutes. Serve garnished with parsley, if you like.
Steamed English spinach and homemade chips are a good accompaniment. To make the chips, scrub 675 g baking potatoes (such as King Edward) and cut into thick chips. Lightly coat the chips by tossing in a plastic bag with a tablespoon of oil and salt and freshly ground black pepper. Heat a baking tray in an oven preheated to 200°C. Spread the chips in a single layer on the hot baking tray and bake for 1 hour, or until browned and crisp, turning a few times during cooking.