In a medium bowl, combine the drained tuna, Greek yogurt or mayonnaise, Dijon mustard, lemon juice, celery if using, red onion, garlic powder, salt, and pepper.Stir until the tuna is evenly coated. Taste and adjust the seasoning as needed.
Slice the avocado and cucumber. Toss avocado slices with a small squeeze of lemon juice to help prevent browning.Toast the bread if desired.
Layer greens on one side of each piece of bread. Add a generous scoop of tuna salad, followed by avocado slices and cucumber.Top with the remaining bread slices.