Prep the dressing: In a medium bowl, whisk Greek yogurt, olive oil, lemon juice, and Dijon until smooth.
Adjust with a splash more lemon if you like it tangier.
Season it: Add a pinch of salt, several grinds of black pepper, and optional garlic powder. Taste and tweak before adding the chicken.
Add the chicken: Fold in chopped or shredded chicken until coated. Aim for bite-size pieces so every forkful gets dressing.
Mix in the crunch: Stir in celery and red onion.
Add herbs and any optional extras like capers or pickles.
Finish and serve: Taste again for salt, pepper, and lemon. Spoon into lettuce cups, pile onto cucumber rounds, or enjoy as is.