Preheat the oven. Set it to 350°F (175°C).Lightly butter a 9-inch square baking dish or a similar 2-quart dish.

Prep the apples. Peel, core, and slice the apples into 1/4-inch thick slices. Thinner slices soften more evenly and bake faster.

Toss the filling. In a large bowl, combine apples with granulated sugar, lemon juice, lemon zest, cinnamon, a pinch of salt, and cornstarch if using. Add vanilla if you like.Stir until the apples look glossy and evenly coated.

Make the crumble topping. In another bowl, mix flour, rolled oats, brown sugar, and 1/4 teaspoon salt. Cut the cold butter into small cubes. Rub the butter into the dry mix with your fingertips or a pastry cutter until you get pea-sized clumps with some sandy bits. Don’t overwork it—you want texture.

Assemble. Spread the apples evenly in the baking dish.Sprinkle the crumble topping over the apples, covering them fully but not packing it down.

Bake. Place the dish on the center rack and bake for 40–50 minutes. The topping should be deeply golden and crisp, and the filling should be bubbling around the edges. If the top browns too quickly, tent loosely with foil.

Rest before serving. Let the crumble sit for 10–15 minutes. This helps the juices thicken slightly so each scoop holds together.

Serve. Spoon into bowls and add a scoop of vanilla ice cream or a dollop of whipped cream.A sprinkle of cinnamon on top is a nice touch.