Mix the celery, mayonnaise, onions, chicken and some salt and pepper until well incorporated. Chill until ready to use.
Slice the croissant in half lengthwise, then slather with butter and grill the croissant in a small pan or on a griddle to crisp it up.
Spread the Mango Chutney Mayonnaise on each side of the croissant. Add the tomato slices to the bottom half of the croissant and season with salt and pepper. Place the shaved lettuce on the tomatoes and add the chicken salad. Add the cheese if using. Place the top of the croissant on the sandwich and cut in half to serve.
Mango Chutney Mayonnaise:
Mix together the chutney, mango if using and the mayonnaise; refrigerate until ready to use.