Prepare the tuna salad:
In a bowl, combine the drained tuna, mayonnaise, and lemon juice. Mix until creamy. Season with salt and pepper to taste.
Assemble the wrap:
Lay a tortilla flat on a clean surface. Place a lettuce leaf in the center.
Add the fillings:
Spoon half of the tuna mixture onto the lettuce, then layer with tomato and onion slices.
Wrap it up:
Fold the sides in and roll tightly from the bottom up. Repeat for the second wrap.
Slice in half diagonally if desired, and enjoy fresh — or wrap in parchment paper for a quick on-the-go meal.