Preheat oven to 350 degrees. Prepare a small casserole dish with non stick cooking spray or butter.
Stat by peeling, coring, and slicing your thinly slicing your apples. Place them in a large mixing bowl. Melt butter and pour over apples. Toss until apples are coated.
Add remaining apple filling ingredients (flour, apple juice, lemon juice, vanilla extract, brown sugar, cinnamon). Toss again until apples are coated. Place all apples in prepared casserole dish.
For your topping add flour, oats, brown sugar, cornstarch, cinnamon, and salt to a mixing bowl. Whisk together until well combined. Add butter and use clean hand to pinch and press until you have a crumbly texture. Add this topping to your casserole dish.
Bake for 30-35 minutes or until topping is golden brown and apples are tender. Let rest for 10-15 minutes then serve.