Place the chicken breasts in a medium saucepan and add enough water to cover by about 1 inch. Bring to a gentle simmer over medium heat, then reduce the heat to low. Cover and cook until the chicken reaches 165°F in the thickest part, about 12–18 minutes depending on thickness. Remove from the water and let cool enough to handle.
Cut the chicken into small bite-size pieces.
In a large bowl, stir together the Greek yogurt, cottage cheese, Dijon mustard, lemon juice, lemon zest, dill, garlic powder, salt and black pepper.
Add the diced chicken, chickpeas and celery. Gently fold until everything is evenly coated with the dressing.
Taste and adjust the seasoning, adding more lemon juice, dill, salt or black pepper as desired.
Serve over arugula or crisp green leaf lettuce, or spoon into a whole wheat pita. Add avocado, if desired.