Place a rimmed baking sheet lined with foil in the oven and preheat to 350°F. Lightly grease a seasoned 10-inch cast iron skillet.
Combine oats, flour, dark brown sugar, cinnamon, and salt in a medium bowl. Cut in butter with a pastry blender until well combined and large crumbs form. Chill in the freezer until ready to use.
Whisk together sugar, arrowroot starch, cinnamon, and salt in a large bowl. Add sliced apples and almond extract. Toss gently to combine. Spoon apple filling into prepared skillet and sprinkle evenly with crumble topping.
Place baking dish on preheated baking sheet and bake for 55 to 60 minutes, until golden brown and filling is bubbling. Cool crisp on a wire rack for at least 30 minutes before serving (cooling for 1 hour is best).
Store leftover crisp tightly covered (or in an airtight container) in the refrigerator up to 2 days. Reheat in a warm oven (350°F) for 15 to 20 minutes, until heated through.
