Season the chicken breasts with garlic powder, salt, and black pepper.
Grill or pan-sear chicken over medium-high heat for 5 to 7 minutes per side until the internal temperature reaches 165°F (74°C).
Remove chicken from heat and let it rest for 5 minutes before dicing into 1/2-inch cubes.
In a mixing bowl, combine the Greek yogurt, Dijon mustard, and lemon juice until smooth.
Add the diced chicken, celery, and red onion to the yogurt mixture and fold until thoroughly coated.
Gently stir in the chopped parsley.
Arrange the mixed greens on plates and top with the chicken salad mixture.