Preheat the oven: Preheat the oven to 375°F (190°C). Butter an 8x8-inch baking dish or a 2–2½ quart baking dish.
Make the oat topping: To a medium-sized mixing bowl, combine the flour, oats, brown sugar, granulated sugar, baking powder, cinnamon, and salt. Pour the melted butter over and mix until evenly moistened and clumps begin to form. Place in the refrigerator while you bring together the apple filling.
1 cup all-purpose flour, 1/2 cup old-fashioned rolled oats, 1/3 cup light brown sugar, 1/3 cup granulated sugar, 1 tsp. baking powder, 1 tsp. ground cinnamon, 1/2 tsp. salt, 1/2 cup cold, unsalted butter
Prepare the apple filling: In a large bowl, combine the apples, granulated sugar, brown sugar, cinnamon, nutmeg, vanilla, lemon juice, flour (or cornstarch), and salt. Toss until the apples are evenly coated.
2½ lbs. apples, 1/4 cup granulated sugar, 1/4 cup light brown sugar, 1½ tsp. ground cinnamon or apple pie spice, Pinch of freshly ground nutmeg, 1 tsp. vanilla extract, 2 tsp. fresh lemon juice, 2 Tbsp. all-purpose flour or cornstarch, Pinch of salt
Transfer filling: Transfer the filling to the prepared baking dish and spread it into an even layer.
Assemble: Remove the oat topping from the refrigerator and scatter evenly over the apples.
Bake: Bake for 35-40 minutes, or until the topping is golden brown and the filling is bubbling at the edges. Be sure to check around the 25-minute mark, and cover with foil if the crumble is browning too quickly. Start Timer
Cool and serve: Let the crisp cool for at least 15 minutes before serving (if you can wait, it smells sooo good!). Enjoy warm with vanilla ice cream!
Vanilla ice cream