In a small mixing bowl, break up the tuna with a fork. Toss with celery, lemon juice, and mayonnaise as needed until the tuna is to your desired creaminess. Season with pepper to taste. Stir to combine.
Lightly toast the bread.
Assemble sandwich: Layer the bread with lettuce, tomato, Tuna Salad, and onion. Close the sandwich with top piece of bread. Cut in half.
Make the lemonade: Mix all ingredients for the lemonade. Serve over ice if desired.
Serve: Enjoy the sandwich with chips, pickle, and homemade lemonade.