Preheat the oven and cook the fish fingers as per the instructions on the packet, either grill or oven cook, as you prefer. Cook until golden and with a good crust. Remove from the heat.
To serve, mix the mayonnaise in a small bowl with the crème fraîche or sour cream and add the capers and a pinch of salt.
Divide the lettuce leaves among the buns, add the fish fingers, add the sliced gherkins and a big spoon of the caper mayonnaise dressing. Serve immediately, with plenty of napkins for everyone.