Wash the cucumber very well, then pat it dry with a clean towel.
With the help of a knife, cut the cucumber lengthwise into 2 halves, then cut each half into 2. In total you should get 4 cucumber boats.
Scoop out of the cucumber all seeds to make space for the tuna.
In a medium bowl, combine drained tuna with Greek yogurt, mayonnaise, diced onion, fresh dill, garlic powder, salt and pepper. Stir all ingredients together until you get a creamy tuna salad. Taste and adjust with more salt and pepper if necessary.
Scoop tuna salad into the prepared boats and serve immediately.