8 tablespoons unsalted butter – cold, cut into small cubes
2.5 pounds apples – peeled, cored, sliced 0.25-inch thick
0.25 cup granulated sugar
1 tablespoon lemon juice
1 tablespoon all-purpose flour
0.5 teaspoon ground cinnamon
0.25 teaspoon kosher salt
vanilla ice cream – for serving
Instructions
Heat the oven to 350°F (175°C). Have a 2-quart baking dish (about 9-inch square) ready.
Make the topping: In a bowl, whisk together 1 cup rolled oats, 0.75 cup all-purpose flour, 0.5 cup light brown sugar, 1 teaspoon ground cinnamon, and 0.25 teaspoon kosher salt.
Add 8 tablespoons cold, cubed unsalted butter to the bowl. Cut it into the dry mixture with a pastry blender or your fingertips until the butter is in pea-sized bits and the mixture is clumpy. Refrigerate the topping while you prepare the apples.
Prepare the filling: In a large bowl, combine the sliced apples with 0.25 cup granulated sugar, 1 tablespoon lemon juice, 0.5 teaspoon ground cinnamon, 0.25 teaspoon kosher salt, and 1 tablespoon all-purpose flour. Toss until the apples are evenly coated.
Spread the apple mixture into the baking dish, pressing gently to level it.
Evenly sprinkle the chilled topping over the apples, breaking up any large clumps and covering the surface from edge to edge. Press lightly so it adheres.
Bake until the topping is deep golden brown and the juices are bubbling thickly in the center, 45–50 minutes. If the top browns too quickly, tent loosely with foil for the last 10 minutes.
Cool on a rack for 15 minutes to let the juices thicken, then serve warm, optionally with vanilla ice cream.