Preheat your oven to 350° F.
In a large bowl, combine oat flour, AP flour, cinnamon, nutmeg, ginger (if using), brown sugar and oats. With your hands or pastry cutter, mix in cold butter with flour mixture until it forms crumbs.
Cut ½ inch off the tops of the apples and save. Remove the core/seeds and discard. Scoop out the apple and keep flesh, leaving a 2 cm border around the edge. A melon baller is a good tool for this.
Dice scooped out apple and remaining apple tops and combine with the crumble mixture.
Place apples on a lined baking pan and press crumble mixture (about 1/3 cup per apple) into the apples and evenly over the top.
Bake for 45 minutes or until apples are tender and the crumble is golden.
Serve warm or at room temperature.