1. Remove a thin strip from back of each celery rib with a sharp knife to create a flat bottom. Cut celery ribs diagonally crosswise into thirty-two 3/4-inch-long pieces.
2. Halve apple lengthwise and core. Slice apple into 1/8-inch-thick slices, then cut enough slices into 1-inch-long matchsticks to measure 1/2 cup. Gently fold apple together with remaining ingredients in a medium bowl until just combined. Season with salt and pepper to taste.
3. Top each piece of celery with a mound of apple mixture and garnish with celery leaves. Makes 32 appetizers.