Heat your oven: Preheat to 350°F (175°C). Butter a 9-inch square baking dish or similar 2-quart dish.

Prep the apples: Peel, core, and slice apples about 1/4-inch thick. You want even slices so they cook at the same rate.

Toss the filling: In a large bowl, combine apples with 1/3 cup granulated sugar, 1–2 tablespoons flour (or 1 tablespoon cornstarch if apples are very juicy), 1 teaspoon cinnamon, a pinch of nutmeg, the zest of half a lemon, and 1 tablespoon lemon juice. Add 1 teaspoon vanilla if using. Stir until apples are evenly coated.

Make the crumble: In another bowl, mix 1 cup flour, 3/4 cup rolled oats, 1/2 cup packed brown sugar, and 1/2 teaspoon salt. Cut 1/2 cup (1 stick) cold unsalted butter into small cubes. Work the butter into the dry mix with your fingertips or a pastry cutter until it forms coarse crumbs with some pea-sized bits.

Assemble: Spread the apples evenly in the baking dish. Sprinkle the crumble topping over the apples, covering them fully but without packing it down.

Bake: Place the dish on a baking sheet to catch drips. Bake for 40–50 minutes, until the topping is golden and crisp and the apple juices are bubbling at the edges. If the top browns too quickly, tent loosely with foil for the last 10 minutes.

Rest and serve: Let the crumble cool for at least 15 minutes so the juices thicken. Serve warm with vanilla ice cream or a spoonful of softly whipped cream.