Topping
1/2 C (70g) all-purpose flour
1/2 C old fashioned oats
1/2 C (110g) packed light-brown sugar
1/2 tsp baking powder
1/4 tsp ground cinnamon
1/4 tsp salt
1/3 C (76g) unsalted butter , diced into small cubes

Apple filling
2 lbs Granny Smith apples (at room temperature) peeled, cored and sliced thin (about 1/8-inch)
3 T (42g) unsalted butter , melted
2 T all-purpose flour
3 T water
1 T lemon juice
1/2 tsp vanilla extract
1/4 C (55g) light-brown sugar
1/2 tsp ground cinnamon
1 pinch salt