Preheat the oven to 350°F (175°C). Grease a 9-inch (23-cm) square baking dish.
In a medium saucepan, bring the apple cider to a boil over medium-high heat. Reduce the heat to medium-low and simmer for 10 minutes, or until the cider is reduced by half. Remove from the heat and set aside.
In a medium bowl, combine the apples, flour, oats, brown sugar, cinnamon, and nutmeg. Add the melted butter and vanilla extract and mix until well combined.
Spread the apple mixture into the prepared baking dish. Pour the reduced cider over the top and sprinkle with the walnuts, if using. Bake for 30 minutes, or until the top is golden brown and the apples are tender.
Allow to cool for 10 minutes before serving. Serve with a scoop of ice cream, if desired.
Interesting Facts
Apple cider is a type of unfiltered apple juice that has a slightly tart flavor and is often used for baking.
The Granny Smith apple is a tart green apple that is perfect for baking because it holds its shape when cooked.
Crisps and crumbles are similar desserts but crisps use a streusel-like topping made with oats, while crumbles use a topping made with flour, sugar, and butter.