Add the brown sugar, melted butter, flour, vanilla, and cinnamon. Toss with the apples until well combined.
¼ cups brown sugar, 3 tablespoons unsalted butter, 2 tablespoons all-purpose flour, ½ teaspoon pure vanilla extract, ½ teaspoon ground cinnamon
In a small bowl, stir together the flour, rolled oats, brown sugar, and cinnamon. Use a pastry cutter, two forks, or your hands to incorporate the cold butter.
½ cups all-purpose flour, ½ cups old-fashioned rolled oats, ½ cups brown sugar, ½ teaspoon ground cinnamon, ⅓ cups unsalted butter
Sprinkle the oat topping over the apples. Bake for 40-45 minutes until the topping is golden brown and the apples are tender.