Combine the sardines, celery, red onion, mayonnaise, dill, lemon juice, lemon zest, mustard, 1/4 teaspoon salt, and pepper to taste in a large bowl. Mash well with a fork.
Sandwich the sardine salad between the bread slices along with the lettuce, tomato, cucumber and sprouts.
Photograph by Kang Kim
Courtesy of Food Network Magazine