01 - Preheat oven to 350°F (175°C) and lightly grease a 9-inch square or similar-sized baking dish.
02 - In a large mixing bowl, toss sliced apples with granulated sugar, lemon juice, ground cinnamon, ground nutmeg, and all-purpose flour until evenly coated. Arrange the mixture in the greased baking dish.
03 - In a separate bowl, combine rolled oats, all-purpose flour, light brown sugar, ground cinnamon, and salt. Add cubed cold butter and blend with a pastry cutter or fork until the topping forms coarse, crumbly texture.
04 - Sprinkle the oat topping evenly over the prepared apples. Transfer the dish to the oven and bake for 40 minutes, or until the topping is golden and the apples are bubbling and tender.
05 - Remove from oven and cool for 10 minutes before serving. Enjoy warm, optionally paired with vanilla ice cream or whipped cream.