Tear lettuce into bite-sized pieces. Thinly slice the onion and soak it in water, then pat dry.

Toast the sandwich bread until golden brown.

Drain the saba cans and mix the fish with mayonnaise, mustard, lemon juice, salt, and pepper in a bowl.

Spread butter evenly on one side of each bread slice. Layer lettuce, onion, and half of the saba mixture on one slice, then top with another slice to form a sandwich. Repeat for the second sandwich.