Preheat your oven to 350°F and place the rack in the center.

Peel 6 medium apples and slice into ¼-inch pieces.

In a large bowl, combine apple slices with granulated sugar, flour, cinnamon, and lemon juice. Toss gently until each piece is coated.

In a medium bowl, stir together rolled oats, brown sugar, flour, cinnamon, and salt until homogenous.

Add cold cubed butter and blend until pea-sized crumbs form.

Spread the apple mixture evenly in a greased baking dish; sprinkle oat topping over the fruit.

Bake for 40–45 minutes, until the topping is golden brown and the filling bubbles.

Let it cool for about 10 minutes before serving.