Drain the can of tuna in water thoroughly. Rinse the 2 large romaine lettuce leaves and pat them dry with paper towels. Finely mince the quarter red onion and cut the half cucumber into small cubes.
Get a mixing bowl and add the drained tuna. Break up the fish pieces a bit with a fork. Add the finely minced red onion, the cucumber cubes, and the tablespoon of sugar-free mayonnaise. Mix everything together until a cohesive salad forms.
Lay the dry romaine lettuce leaves with the curved side facing down next to each other on the work surface. Spoon the freshly mixed tuna salad into the leaves and distribute the filling evenly between these two cups.
Carefully place the filled lettuce cups on a plate. Serve immediately so the lettuce retains its crispy texture.