Rinse chicken and pat dry with a paper towel. Season both sides of chicken breasts with salt and pepper.
Grill over medium-high heat for 5-7 minutes per side, or until fully cooked. Transfer chicken to a cutting board and let cool slightly.
Once cool, use 2 forks to shred the grilled chicken breasts. (Another option is to dice the chicken into ½”-1” cubes.)
For the Chicken Salad
Using a large bowl, combine all ingredients except for croissants and lettuce (shredded chicken, celery, onion, garlic, mayonnaise, sour cream, mustard, parsley, salt and pepper).
Refrigerate until cold.
Serve chicken salad on sliced croissants with lettuce and tomato {optional}.