Preheat the oven to 425˚F (220˚C). Line a baking sheet with parchment paper.
Arrange the bread in a single layer on the prepared baking sheet. Brush each piece of bread with olive oil. Lightly sprinkle with salt. Bake for 12-14 minutes, until the edges are light golden brown. Let cool.
Make the Waldorf chicken salad: In a medium bowl, combine the chicken breast, grapes, celery, mayonnaise, and lemon juice and mix well. Season with salt and pepper.
Place 2 apple slices on each crostini. Top with 1 heaping tablespoon of chicken salad. Garnish with ½ teaspoon chopped walnuts.