Preheat oven to 350°F and grease a 9x13-inch pan. Simple step, one less thing to worry about.
Toss apples with granulated sugar, cinnamon, and lemon juice. Set aside while you make the crust.
Mix flour, oats, brown sugar, baking powder, and salt in a bowl. Add cold butter and use a fork or your fingers to make coarse crumbs.
Press half the crumbs into the pan to form a base. Scatter the apple mixture over it.
Sprinkle remaining crumbs evenly on top. Bake 35–40 minutes until golden and apples are bubbly.
Prep time: 15 minutes | Cook time: 35 minutes | Servings: 12