Remove pan from heat and mix in egg yolks, sugar, lemon juice, and cinnamon. Fill a partially baked crust with the apple mixture, and bake for 30-35 minutes, or until the filling is set. Let cool completely on a wire rack.
In the top of a double broiler, combine egg whites, sugar, and cream of tartar. Cook over medium heat, stirring constantly until sugar is dissolved. Remove from the heat and beat with an electric mixer until stiff peaks form.