Briefly toast the rolls in the oven, then cut them open.
Heat the oil in a pan and fry the fish fingers in it. Cut the onions into small rings, turn them in flour and then roast them in a second pan until golden brown.
Mix the mustard, dill, sugar and tartar sauce into a sauce and add lemon juice to taste. Brush all halves of the burger with it.
Cut the cucumber and tomato into slices.
Cover each lower half of the burger with 1 lettuce leaf, some of the onions, 2 fish fingers and one slice of cucumber and one tomato slice. Fold the burger halves together and serve the burgers.