PREP: Line a sheet pan with a Silpat liner or parchment paper (just in case anything bubbles over) and lightly grease an 8x8 or 9x9-inch baking pan (or 2-quart pan). Set aside.
TOPPING: In a large bowl, combine flour, oats, sugar, cinnamon, baking soda, and salt. Cube the butter and mix it into the dry ingredients. Use a pastry cutter to blend, then press with fingertips until the mixture becomes crumbly. Refrigerate while preparing the filling.
FILLING: Peel and chop apples into 1/2-inch pieces. In a large bowl, mix apples with lemon juice, sugar, cinnamon, cornstarch, and vanilla until evenly coated.
ASSEMBLY: Place the apple filling and any juices into prepared pan. Gently press down and spread apples evenly. Evenly sprinkle the crumble on top without pressing it down. Chill the dish for 20 minutes. Meanwhile, preheat the oven to 350°F. After chilling, place the dish on the lined pan.
BAKE: Bake for 40-50 minutes, or until topping is golden, apples are fork tender, and the filling is very slightly bubbling around the edges.. If topping browns too quickly, cover with foil.
SERVE: Let crumble cool for 15-20 minutes to allow apples to continue to thicken. Serve warm with ice cream or whipped cream.
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Notes
Note 1: Apples: Weigh before peeling to get 2 pounds. We 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.