Preheat your oven to 350°F (175°C). Lightly grease a 10-inch cast iron skillet with a small amount of coconut oil or butter.
Peel, core, and dice the apples into roughly ½-inch chunks. Toss immediately with lemon juice to prevent browning and enhance the natural apple flavor.
In a large bowl, combine the rolled oats, flour, baking powder, cinnamon, nutmeg, and salt. Whisk together to ensure even distribution.
In a separate bowl, whisk together the eggs, maple syrup, melted coconut oil, vanilla extract, and milk until well combined.
Pour the wet ingredients into the dry ingredients and stir until just combined. Fold in the diced apples gently, reserving about ¼ cup for topping.
In a small bowl, mix together the oats, almond flour, coconut sugar, and cinnamon for the topping. Add the chilled coconut oil or butter and work it into the dry ingredients with your fingertips until the mixture resembles coarse crumbs.
Pour the batter into your prepared skillet, spreading it evenly. Scatter the reserved apple pieces on top, then sprinkle the crumble mixture evenly over everything.
Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean with a few moist crumbs.
Allow the cake to cool in the skillet for at least 15 minutes before serving.