Drain the canned tuna very well — press it in a fine-mesh sieve to remove every bit of liquid so the filling isn't watery. Flake the tuna apart with a fork in a mixing bowl.

Stir the Greek yogurt, Dijon, lemon juice, red onion, celery (if using), garlic powder, salt, and pepper into the flaked tuna. Taste and adjust salt — the filling should be creamy and well seasoned.

Spread about 1/2 cup of the tuna mixture over one half of each tortilla, leaving a 1/2-inch border. Sprinkle 1/2 cup of the shredded cheese over the filling on each tortilla, then fold the empty half over to close. Don't overfill — a thin, even layer melts better and flips without spilling.

Melt 1/2 tablespoon of butter in the large skillet over medium heat. Cook the quesadillas one or two at a time (don't crowd), 2–3 minutes per side, until the tortilla is deep golden and crisp and the cheese is fully melted. Transfer finished quesadillas to a wire rack or a 200°F oven to stay crisp while you finish the batch.

Add the remaining butter and cook the rest of the quesadillas the same way. If the tortilla is browning too fast but the cheese isn't melted, lower the heat — medium, not high, is the whole trick.

Cut each quesadilla into 3 wedges. Serve hot with salsa, sour cream, or hot sauce.