In a small saucepan, heat pie filling on low. Meanwhile, in a large bowl, beat egg whites and cream of tartar on medium speed until soft peaks form. Gradually beat in sugar, 1 tablespoon at a time, on high until stiff glossy peaks form.
Remove pie filling from the heat; stir in extract. Pour into pastry shell. Spread meringue over hot filling, sealing edges to crust. Sprinkle with walnuts.
Bake at 350° for 15 minutes or until meringue is golden brown. Cool on a wire rack for 1 hour. Chill for at least 3 hours before serving. Refrigerate leftovers.