Preheat the oven to 350°F (175°C). Grease the 9x13 inch baking dish.
In a mixing bowl, combine the diced apples, ground cinnamon, and granulated sugar. Mix well and spread the apple mixture evenly in the prepared baking dish.
In another bowl, mix the rolled oats, all-purpose flour, brown sugar, melted butter, baking powder, and salt until crumbly. If using, stir in the chopped nuts.
Sprinkle the crumble mixture evenly over the apples in the baking dish.
Drizzle the milk over the top of the crumble.
Bake in the preheated oven for 30 minutes or until the top is golden brown and the apples are tender.
Remove from the oven and let it cool slightly before serving.