Preheat your oven to 375 degrees. Spray a 9x13 dish with cooking spray.

In a medium-sized bowl, mix together flour, old fashioned oats, brown sugar, baking powder, cinnamon, and salt to make your crumb topping.

Next, add the diced butter. Using a pastry cutter or your hands, blend the butter into the dry mixture until it's well combined and crumbly. Place the mixture in the refrigerator to cool while you prepare the apple filling.

In a bowl, whisk together the melted butter and flour. Then, add the water, lemon juice, and vanilla extract. Finally, add cinnamon and brown sugar. Be sure to mix well so there are no chunks of brown sugar.

Place your apples in your 9x13 baking dish. Pour the butter mixture over the apples and toss to coat the apples well. Make sure to spread your apples in the dish evenly once coated.

Remove the crumb topping from the refrigerator and sprinkle evenly across the apples that are at the bottom of the pan.

Bake in your preheated oven for about 35-40 minutes or until the top is brown and the apples are tender.

Let the apple crisp cool for ten minutes. Serve with vanilla ice cream and a drizzle of caramel sauce, if desired.