1Drain the tuna in a strainer in the sink. Chop the celery to measure 1/2 cup. Peel and chop the onion to measure 1/4 cup.
2In a medium bowl, mix the tuna, celery, onion, mayonnaise, lemon juice, salt and pepper.
3Spread tuna mixture on 4 bread slices. Top with remaining bread slices.