Prepare the sauce. Pour mayonnaise into a bowl, add cucumbers, mustard powder, dill, parsley, onions, lemon juice, and capers (optional). Stir well, cover, and refrigerate until serving.
Lightly beat the egg with 1 teaspoon of water. Pour bread crumbs into another dish. Cut the fillet into 10-15 cm pieces. Dip each piece of flounder on all sides in a beaten egg and then in bread crumbs.
Heat 1 tablespoon of oil in a deep saucepan, put fillet pieces, and fry for about 6 minutes on both sides. Transfer the fish to a paper towel.
Brush the slices of bread on one side with the remaining butter. Put lettuce leaves on the bread, fish on top, and cucumber sauce on top. Decorate the sandwiches with lemon slices and dill sprigs.