Combine drained pineapple and mandarin oranges in a bowl; cover with plastic wrap and refrigerate 8 hours to overnight.

Stir mayonnaise and mustard together in a bowl; add chicken, celery, mushrooms, green bell pepper, olives, and onion. Stir mixture until evenly mixed. Cover bowl with plastic wrap and refrigerate 8 hours to over night.

Stir pineapple and oranges into the chicken mixture to coat. Spoon into lettuce leaves and top with croutons to serve.