Dice the avocado into 1/2 inch pieces, put in a bowl large enough to hold the whole salad, and toss with one tablespoon of lime juice.
Peel strips from the cucumber if desired, then cut into fourths lengthwise and slice into bite-sized pieces.
Chop the cilantro and thinly slice the green onion.
Whisk together the mayo and lime juice to make the dressing.
Add the chopped cucumbers, chopped cilantro, and sliced green onions to the avocado and gently combine. Add the dressing and stir just enough to combine.
Drain the tuna and use a fork to partly break apart the tuna into chunks.
Stir tuna into the salad and GENTLY combine (don't over-stir; you want the tuna to remain chunky.)
Season to taste with salt and fresh-ground black pepper and serve.
