Preheat oven to 375°. Mix flour, baking powder and, if desired, salt. In a small bowl, whisk together egg yolks, milk and lemon juice; stir into flour mixture. , In another bowl, beat egg whites on medium speed until foamy. Gradually add sugar, 1 tablespoon at a time, beating on high after each addition until stiff peaks form. Fold into flour mixture., Pour into a 9-in. deep-dish pie plate coated with cooking spray. Arrange apple slices over top. Mix sugar and cinnamon; sprinkle over apple., Bake, uncovered, until a knife inserted in the center comes out clean, 18-20 minutes. Serve immediately.