Preheat oven to 375 degrees. Lightly grease a cast iron pan (or casserole dish if you don’t have one) and set aside.
In a bowl, whisk together melted butter and flour until well combined and then add water and vanilla. Once combined, add in brown sugar, cinnamon, nutmeg and pinch of salt.
Pour mixture over thinly sliced apples. Then, pour apple mixture into greased baking dish and spread into an even layer. Set aside.
Combine the oats, flour, brown sugar, baking powder, baking soda and melted butter together. Crumble evenly over the apple mixture.
Bake until top is golden brown and apples are tender when pierced with a toothpick. About 40 minutes.
Remove from oven and let it sit for 10 minutes. Serve warm with ice-cream and caramel!