1. Preheat the oven to 220°C, fan 200°C, gas 7. Put the fish on a baking tray and bake for 20 minutes, until crispy and golden.
2. Meanwhile, butter the brioche rolls and line each with lettuce leaves. Put the mayonnaise in a bowl and add the chopped gherkins for crunch, the pickle juice and the parsley. Season with black pepper and mix until well combined.
3. When the fish is ready, cut into chunky pieces and place inside the lettuce leaves. Top the fish with some of the creamy tartare sauce and then sprinkle over the crumbled crisps for a vinegary tang.