Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish.

In a large bowl, toss the sliced apples with lemon juice, granulated sugar, cinnamon, and cornstarch until evenly coated. Set aside.

In another bowl, combine oats, flour, brown sugar, and salt. Use a pastry cutter or your fingertips to work in the cold butter until the mixture forms pea-sized clumps.

Spread the apple mixture in the prepared baking dish. Sprinkle the oat topping evenly over the apples.

Bake for 40–45 minutes, or until the topping is golden brown and the apples are bubbling around the edges.

Let the apple crisp cool for 10 minutes before serving. Top with a scoop of vanilla ice cream if desired.