1. Heat oven to 350 degrees. Use a 2-inch-round cookie cutter to cut about 4 dozen rounds from the brioche slices. Place rounds on a baking sheet; transfer to oven until they are barely golden, about 7 minutes, turning over halfway through cooking time.

2. Place the chicken stock in a medium saucepot, and bring to a boil. Add the chicken breast, and reduce to a simmer. Cook for about 15 minutes, until the chicken is cooked through. Drain, reserving the stock for another use, and set the chicken aside to cool. When it is cool enough to handle, shred the chicken into bite-size pieces.

3. Peel and core the apple, and cut into 1/4-inch dice. Place diced apple in a large bowl, and toss with lemon juice to coat. Add the shredded chicken, celery, mayonnaise, tarragon, salt, and pepper.

4. When ready to serve, top each brioche toast with 1 heaping teaspoon chicken salad, and garnish with a pinch of freshly chopped tarragon.