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