In a small nonstick skillet, sauté apple and onion in butter until tender. Remove from the heat; stir in the blue cheese, 3 tablespoons of walnuts, and salt. Spoon a rounded tablespoonful into each tart shell.
Place on an ungreased baking sheet. Bake at 350 degrees for 5 minutes. Sprinkle with remaining walnuts; bake 2-3 minutes longer or until lightly bronwed.