Preheat oven to 350F degrees. Butter an 8×8 baking dish, or spray with non-stick cooking spray. Set aside.
Peel, core, and cut the apples into ½-inch thick slices.
In a large bowl, stir to combine sliced apples, ⅓ cup brown sugar, ½ teaspoon cinnamon, nutmeg, 1 tablespoon flour, 8 tablespoon cold cubed butter, and lemon juice.
Set your apples aside.
To make your topping, in a food processor, combine your old-fashioned rolled oats, ¾ cup all-purpose flour, ½ cup packed light brown sugar, 1 teaspoon ground cinnamon, ½ teaspoon salt, and ¾ cup cold unsalted butter cut into small cubes.
Pulse on low until small bits form.
Top your apple mixture with your crumble.
Bake for 40-45 minutes or until your apple are tender and topping is golden.