Prepare the tuna salad by combining the tuna, avocado, red onion, garlic powder, lemon juice, salt, and pepper. Stir together to fully break up the tuna and mash the avocado. Set aside.

Wash the cucumber well, slice off the ends, and slice it in half vertically. Scoop out the inside of the cucumber until the cucumber is fully hollowed out.

Spoon the tuna salad into the cucumber boats and top with green onion. Serve fresh!