Step 1: In a large mixing bowl, combine the shredded or diced cooked chicken, finely diced celery, and finely minced red onion.
Step 2: Add the mayonnaise, fresh lemon juice, salt, and black pepper to the chicken mixture. Start with about 3/4 of the mayonnaise and add more if needed to reach your desired creaminess.
Step 3: Mix all ingredients thoroughly until the chicken is well coated and the salad is creamy. Be careful not to overmix, as this can break down the chicken too much.
Step 4: Taste the chicken salad and adjust seasonings (salt, pepper, lemon juice) as needed. For best flavor, cover the bowl and refrigerate for at least 30 minutes, allowing the flavors to meld.
Step 5: When ready to serve, gently slice each croissant horizontally, almost but not entirely through, creating a hinge. You can lightly toast the croissants if desired for extra warmth and crispness.
Step 6: If using, place a piece of butter lettuce inside each croissant. Spoon a generous amount of the chilled chicken salad into each prepared croissant.
Step 7: Serve immediately and enjoy your delicious Classic Chicken Salad Croissant Sandwich!