Peel, core and slice the apples into ¼” slices. Sprinkle sliced apples immediately with a bit of lemon juice and toss until coated.
In a large bowl, mix together all the filling ingredients except for the apples.
Add the apples to the filling mixture and mix until well coated.
Make the Topping:
Combine all dry ingredients in a bowl, except for the butter. Whisk together.
Add the softened cubed butter and pinch it together with the dry ingredients until a softened crumbly mixture forms. It should look moist and not dry. Set aside.
Assembly:
Preheat the oven to 350°F and grease an 8” or 9” cast iron skillet or baking pan with oil or cooking spray.
Spread the apple filling mixture into the pan.
Sprinkle the apples with all of the oat topping.
Bake for 35-45 minutes or until golden brown and the apples are tender and cooked through.