1. If using regular yogurt, place in a strainer lined with a paper towel. Put strainer over a bowl and place in the refrigerator to drain and thicken for 30 minutes.
2. In a small bowl, combine the thickened or Greek-style yogurt, mayonnaise, lemon juice, mustard, salt, and thyme; mix until smooth. Fold in chicken, grapes, walnuts, and apples. Season with pepper.
3. Place one lettuce leaf on a wrap. Spoon about 3/4 cup of the chicken filling onto wrap and roll wrap around filling.