Empty tuna into a fine-mesh strainer for 3 to 5 minutes, then press gently with a spoon. It should remain moist but release no pooling liquid.
Finely chop celery, red onion, pickles and herbs. Blot juicy pickles dry; rinse and thoroughly dry the onion if a milder flavor is preferred.
Stir Greek yogurt, Dijon, lemon juice, garlic powder and black pepper in a medium bowl. Fold in celery, onion, pickles and herbs.
Fold in the drained tuna, breaking up only the largest chunks. Stop while some distinct flakes remain.
Refrigerate for 10 minutes so the tuna absorbs some dressing and the flavors settle.
Stir once. Add Greek yogurt 1 tablespoon at a time if dry, then season with salt, pepper or a few drops of lemon to taste. Serve chilled.
