Preheat the oven to 375°F (190°C).

Grease a 9-inch deep-dish pie plate with cooking spray.

In a mixing bowl, combine the flour, baking powder, and salt (if using).

In a separate small bowl, whisk together the egg yolks, milk, and lemon juice, then stir this mixture into the flour mixture.

In another bowl, beat the 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 the beaten egg whites into the flour mixture.

Pour the batter into the prepared pie plate.

Arrange apple slices over the top.

Mix sugar and cinnamon, then sprinkle over the apples.

Bake, uncovered, until a knife inserted near the center comes out clean, for about 18-20 minutes.

Serve immediately.