Prepare the cucumbers: Wash and dry the cucumbers thoroughly. Slice them into rounds about ½ inch thick. If the cucumbers are especially watery, gently pat the slices dry with paper towels. Arrange them on a serving plate.
Drain the tuna: Open the tuna cans and drain away all excess liquid. Transfer the tuna to a medium mixing bowl and use a fork to flake it into small pieces.
Make the creamy tuna filling: Add the mayonnaise, Greek yogurt, lemon juice, Dijon mustard, chopped red onion, parsley or dill, garlic powder, black pepper, and salt to the tuna. Add the chopped celery and capers if desired.
Mix until creamy: Stir everything together until well combined. The filling should be creamy but thick enough to sit on top of the cucumber slices without sliding off. Taste and adjust the lemon juice, salt, and pepper as needed.
Assemble the cucumber bites: Spoon approximately 1 tablespoon of tuna mixture onto each cucumber round. For a more elegant presentation, transfer the tuna salad to a piping bag or a zip-top bag with the corner snipped off, then pipe it onto the cucumber slices.
Garnish and serve: Sprinkle lightly with paprika and add a small sprig of fresh dill or parsley. Serve immediately for the best crunch, or refrigerate briefly before serving.
