Preheat oven to 180C (350 F).
Grease a 25 cm (10inch) cast-iron skillet with oil and line bottom with parchment paper. Or grease and flour 25 cm springform pan.
Sift together flour and cornstarch a few times. In a large mixing bowl, stir together flour and starch mixture, baking soda, baking powder, cinnamon, and salt. In a separate bowl, whisk together eggs, granulated sugar, and brown sugar, until frothy around 2 minutes. Whisk in oil, vanilla extract, and apple sauce.
With a wooden spoon or rubber spatula slowly incorporate dry ingredients, mixing well with each addition.
Stir in chopped nuts and apples. Pour into prepared skillet or springform pan and spread evenly. Top with sliced apples, if desired.
Bake for 45-50 min, or until a toothpick comes out dry. Rotate the pan once or twice during baking. If the top starts to brown too fast, cover with aluminum foil.
Remove from oven and let cool completely before topping with glaze and more chopped nuts.