Rinse cucumbers. Peel off skin every ¼ inch, all the way around. Cut lengthwise. Scoop out the seeds with a small spoon.
Rinse lemon. Zest using the small holes of a box grater. Cut in half. In a small bowl, squeeze juice. Discard seeds.
Rinse and chop green onions.
Drain tuna. In a colander, drain and rinse beans.
In a medium bowl, mash beans lightly with a fork.
Add green onions, tuna, oil, mustard, salt, pepper, lemon zest, and 2 Tablespoons of the lemon juice to beans. Mix with a fork.
Fill each cucumber half with ¼ tuna mixture. Serve.