1. Combine celery, walnuts, and 3 tablespoons of the mayonnaise in a medium bowl.
2. Hollow out apple halves with a melon bailer or a spoon, discarding cores. Dice removed apple flesh and add to celery mixture. If mixture looks dry, add remainingtablespoon mayonnaise.
3. Set each apple half on a plate. Fill apples with salad mixture.