Drain the tuna thoroughly, then add it to a large bowl with the celery, red onion, and dill relish. The mixture should look well flaked and fairly dry, with the vegetables evenly scattered through the fish.

In a small bowl, whisk together the mayonnaise, lemon juice, garlic powder, dried dill, and salt and pepper. You want a smooth, creamy dressing so the seasonings are fully distributed before they hit the tuna.

Add the dressing to the tuna mixture and fold everything together until coated. Stir gently so the tuna keeps some meaty flakes instead of turning into a paste.

Taste and adjust with more salt and pepper or mayonnaise if needed. The salad should taste savory and tangy, and the texture should be creamy enough to hold together easily.

Serve right away, or cover and refrigerate for 3 to 5 days. After chilling, give it a gentle stir so any settled moisture blends back into the dressing.