Remove the skin from the chicken. Peel meat from the bones and shred. Do not cut with a knife.
Peel and core the apple. Cut into 1/2-inch cubes.
Cut the celery stalk into 1/4-inch cubes.
Mix chicken, apple, celery, capers, and mustard. Add enough mayonnaise to coat lightly.
Taste and correct seasoning with salt, pepper, and lemon juice. Add more mustard if desired.
Season lettuce with mustard vinaigrette and place it in the center of a serving platter. Top with chicken salad and garnish with tomatoes, green beans, and slices of hard-boiled egg.