Method

Position a rack in the center of the oven and preheat to 220 degrees C. Peel and core the apples, then cut into 1.5cm thick slices. Place apple slices on a 45cm by 33cm sheet pan and toss with the lemon juice, 1/4 cup of the brown sugar, 2 tablespoons of melted butter, 1 teaspoon cinnamon, and 1/8 teaspoon salt.

Spread the apples evenly on the pan, cover with foil and bake until the apples have softened and released some liquid, about 20 minutes.
Meanwhile, combine the oats, flour, remaining 1 1/4 cups brown sugar, 1 teaspoon cinnamon, and 1 teaspoon salt together in a large bowl, breaking up any lumps with a fork. Add the remaining melted butter to the oat mixture, stirring with a fork until it is moistened and crumbly.
Carefully uncover the sheet pan and scatter the oat crumble evenly over the top. Bake uncovered until the top is crisp and golden brown and juices are bubbly, about 15 minutes. Serve with ice cream.