Press unbaked pie crust onto bottom and up side of 9-inch quiche pan or pie plate. Flute edge and set aside while preparing Apple Filling.
In a large bowl toss apple slices with butter, sugar, flour, cinnamon, nutmeg and salt. Spoon into pastry-lined pan. Bake at 350 for 30 minutes or until edge of crust is lightly browned. Remove from oven and set aside while preparing Yogurt Topping.
In a medium bowl combine yogurt, egg, sugar, flour, vanilla extract and salt, beating with fork or wire whisk until blended. Pour evenly over hot apple tart. If desired, arrange walnuts over top. Bake at 350 degrees additional 10 minutes. Cool completely before serving.