How to Make It
In a large bowl, whisk the mayonnaise with the mustard, egg, cilantro, scallions and jalapeño. Fold in the fish and the 1/2 cup of panko and season with salt and pepper. Form the mixture into six 3 1/2-inch patties and coat generously with panko.
In a large nonstick skillet, melt the butter in the oil. Add the fish patties and cook over moderately high heat, turning once or twice, until crisp and heated through, about 8 minutes. Set the patties on the buns. Top with lettuce, tomato and pickles and serve right away.