Preheat the oven to 200°C, fan 180°C, gas 6. Line a baking sheet with baking parchment. Cut the fish into 10 pieces measuring about 8-9cm in length and 3cm wide. Squeeze the lemon juice over the fish pieces.
Put the polenta, breadcrumbs and chives on a large flat plate. Put the beaten egg in a shallow bowl. Dip the fish in the egg, then turn several times in the polenta and breadcrumb mixture to coat. Repeat with all the pieces of fish.
Place the crumbed fish on the lined baking tray and drizzle with the olive oil. Bake for 15 minutes, turning halfway through, until crunchy and golden brown.
Serve with mashed potato, cooked peas and sweetcorn.