Remove the white meat from the rotisserie chicken, chop it into bite-sized pieces, and place it in a large mixing bowl.

Add the diced celery, mayonnaise, sour cream, lemon juice, garlic powder, onion powder, salt, and black pepper. Fold everything together with a spatula until the ingredients are evenly coated.

Transfer the mixture to the bowl of a stand mixer and mix on the lowest speed until just combined, stopping occasionally to scrape down the sides with a spatula. Be careful not to overmix. Add more mayonnaise, salt, or pepper if needed.

Gently fold in the halved grapes.

To serve, layer Bibb lettuce on the bottom half of a croissant, spoon on the chicken salad, and top with the remaining half of the croissant.