The tuna, usually from a can, is combined with mayonnaise and often mixed with celery, red onion, and a splash of lemon juice for added flavor and crunch. This mixture is spread onto slices of bread, which can be toasted or left soft, and often accompanied by fresh lettuce, tomato slices, and pickles. The result is a classic, easy-to-make sandwich that's both nutritious and delicious, perfect for a quick lunch or a casual meal.