Dice the chicken into 1/4-inch pieces and set aside. Dice the celery into 1/4-inch pieces and set aside. Toast the walnuts in a dry skillet over medium heat for 3-4 minutes until fragrant, then chop into 1/4-inch pieces. Chop the chives finely and set aside. I like to prep all my components before mixing so the final assembly is quick and nothing gets overworked.

In a medium bowl, whisk together the Greek yogurt, mayonnaise, tarragon, Dijon mustard, lemon juice, salt, and black pepper until smooth and well combined. This dressing base creates a tangy, herbaceous foundation with just the right balance of acidity and creaminess.

Add the prepared chicken, celery, cranberries, toasted walnuts, and chives to the dressing base from Step 2. Fold gently until all components are evenly coated and distributed. I find that folding rather than stirring helps keep the texture from becoming mushy—the ingredients stay distinct and pleasant.

Spoon the chicken salad mixture into each pre-baked phyllo shell, filling them generously but carefully so they don't overflow. Arrange the filled shells on a serving platter and finish with a light dusting of black pepper and a small garnish of fresh chives on top for visual appeal.