In a large bowl, toss apples with lemon juice. In a small bowl, whisk sugar, flour, milk, egg yolks and lemon zest until smooth. Pour over apples and toss to coat. , Line a 9-in. pie plate with pastry; trim to 1/2 in. beyond edge of pie plate and flute edges. Pour filling into crust; dot with butter. Cover edges loosely with foil. , Bake at 400° for 20 minutes. Remove foil; bake 25-30 minutes longer or until apples are tender. Reduce heat to 350°., In a bowl, beat the egg whites and cream of tartar on medium speed until foamy. Gradually beat in sugar, 1 tablespoon at a time, on high just until stiff peaks form and sugar is dissolved. , Spread evenly over hot filling, sealing edges to crust. Bake for 15 minutes or until golden brown. Cool on a wire rack. Store in the refrigerator.