1. Put the breadcrumbs in a bowl. Put the flour and paprika in another small bowl; season. Put the egg into a third small bowl and whisk lightly with a fork.
2. Cut the fish lengthways into 6 long fingers, roughly 2cm wide. Toss in the flour to coat, then dip each individually in the beaten egg, followed by the breadcrumbs. Transfer to a plate and chill. Mix all the tartare sauce ingredients together; season.
3. Pour the oil into a 22cm frying pan so it reaches a depth of about 1cm. Place over a high heat and, when hot, add the fish fingers. Fry for 3 minutes on each side, until golden. Transfer to a plate lined with kitchen paper and blot the excess oil from the crumbs. Sprinkle with salt and squeeze over the lemon juice.
4. Spoon 1 heaped tbsp tartare sauce over the base of each bun, top with lettuce leaves and 2 hot fish fingers, then finish with the bun tops. Eat while still warm