1/2 cup (70g) all-purpose flour
1/2 cup old fashioned oats
1/2 cup (110g) packed light-brown sugar
1/2 tsp baking powder
1/4 tsp ground cinnamon
1/4 tsp salt
1/3 cup (76g) unsalted butter , diced into small cubes
2 lbs Granny Smith apples (at room temperature) peeled, cored and sliced thin (about 1/8-inch)
3 Tbsp (42g) unsalted butter , melted
2 Tbsp all-purpose flour
3 Tbsp water
1 Tbsp lemon juice
1/2 tsp vanilla extract
1/4 cup (55g) light-brown sugar
1/2 tsp ground cinnamon
1 pinch salt
Preheat oven to 375 degrees and position oven rack one level below the center. Butter an 8 by 8 inch baking dish (or small casserole dish with a similar size), set aside.
In a mixing bowl whisk together 1/2 cup flour, the oats, 1/2 cup brown sugar, 1/2 tsp baking powder, 1/4 tsp cinnamon and 1/4 tsp salt for 30 seconds.
Add diced butter and using clean fingertips, rub butter into dry mixture until it comes together into small crumbles. Transfer to refrigerator to chill while preparing filling.
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.
Remove topping from refrigerator and sprinkle into crumbles evenly over top of apples.
Bake in preheated oven until top is golden brown and apples are tender when pierced with a toothpick, about 35 minutes.
Remove from oven and allow to rest 10 minutes before serving. Serve warm with vanilla ice cream and salted caramel sauce if desired.