Line a sheet pan with parchment paper or a silicone baking mat. Lightly grease an 8 x 8 or 9 x 9-inch baking pan.
In a large bowl, combine flour, oats, sugar, cinnamon, baking soda, and salt. Cube the butter and mix it into dry ingredients. Use a pastry cutter to blend, then press with your fingertips until crumbly. Refrigerate while preparing filling.
Peel and cube apples into 1/2-inch pieces. In a large bowl, mix apples with lemon juice, sugar, cinnamon, cornstarch, and vanilla until evenly coated.
Transfer the apple mixture (including juices) to the prepared pan, pressing down evenly. Sprinkle the crumble mixture on top without pressing it down. Chill the dish for 20 minutes. While chilling, preheat oven to 350°F.
After chilling, place the dish on the lined pan. Bake for 40–50 minutes, or until topping is golden, apples are tender, and filling is slightly bubbling around the edges. If topping browns too quickly, cover with foil.
Let cool for 15–20 minutes to allow filling to thicken. Serve warm with ice cream or whipped cream!
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Notes
Note 1: Weigh apples before peeling to get 2 pounds. I like using half Granny Smith and half Honey-crisp for this recipe. Other varieties can work, but less firm apples may create a mushy or watery crumble. Peel apples and cut into 1/2-inch cubes. You’ll get about 5-1/2 to 6 cups apples from 5–7 apples.Storage: To maintain crispness, store unbaked topping and apples separately. For leftovers, cover with plastic wrap with holes and refrigerate. It’s best eaten the same day but good for up to 2 days.