Preheat oven to 350 degrees. Season chicken with salt and pepper. Cook for 20 minutes, or until chicken is cooked through. Refrigerate for about an hour, and then dice the chicken.
Add diced chicken to a medium bowl and add mayonnaise, curry powder, celery, green onion, mango chutney and mix until well combined. Season with salt and pepper to taste. Adjust all flavours as you see fit - add more mayo, curry or chutney.
Cut croissants in half, fill with chicken salad. Dress sandwich with lettuce, mango and red onion.