In a large bowl, combine apples, lemon juice, vanilla extract, granulated sugar, cinnamon, and flour. Gently toss until apples are well coated. Let the apple mixture sit while you prepare the crumble topping.
For the crumble topping: in a large bowl, combine the flour, oatmeal, brown sugar, granulated sugar, salt, and cinnamon. Stir together with a whisk. Mix in the cold butter with your fingers until the mixture comes together and you have a crumbly topping.
Place the apple mixture in a greased 12-inch cast iron skillet or greased 9x13-inch baking pan. Sprinkle the crumble mixture evenly over the apples.
Bake for 40 to 45 minutes, or until the crumbly is browned and bubbling. Serve warm or at room temperature. I recommend serving with vanilla ice cream.