In a mixing bowl whisk together 1/2 cup flour, the oats, 1/2 cup brown sugar, the baking powder, 1/4 tsp cinnamon and 1/4 tsp salt for 30 seconds.
In a small mixing bowl, whisk together melted butter and flour until well blended, then mix in water, lemon juice and vanilla. Stir in 1/4 cup brown sugar, 1/2 tsp cinnamon, and pinch of salt.
Place apples in a large bowl then pour butter mixture over apples and toss to evenly coat, then pour apple mixture into prepared baking dish and spread into an even layer.
Bake in preheated oven until top is golden brown and apples are tender when pierced with a toothpick, about 35 minutes.