> 1/2 cup (1 stick) cold unsalted butter, cubed, plus more for greasing
> 1 cup light brown sugar, divided
> 1 cup old-fashioned oats
> 3/4 cup plus 2 tbsp all-purpose flour
> 2 tsp ground cinnamon, divided
> 3 lbs sweet apples, such as Golden Delicious apples (8-9 apples)
> 1 tbsp lemon juice
To Serve (Optional):
> Caramel sauce
> Vanilla ice cream
Position a rack in the middle of the oven and preheat to 350°F. Grease a 3-qt. baking dish with butter. To a medium bowl, add the cubed butter, ¾ cup brown sugar, oats, ¾ cup flour, 1 tsp cinnamon, and a pinch of salt. Using clean hands or a fork, cut butter into flour mixture. Press topping together to form chunks. Freeze 10 min.
Meanwhile, peel, core, and roughly chop the apples. To a large bowl, add apples, remaining ¼ cup brown sugar, remaining 2 tbsp flour, remaining 1 tsp cinnamon, lemon juice, and a pinch of salt. Toss to combine and add to prepared baking dish. Cover apples with oat topping, keeping large chunks intact.
Bake 40–45 min. on middle rack, until apples are tender and topping is golden brown. Let cool 10 min. before serving. Serve with the caramel sauce and vanilla ice cream, if desired.