01 - Set the oven temperature to 325°F.
02 - Wash and core the apples, keeping the bottom intact to create a cavity. Arrange them in a baking dish.
03 - Combine orange zest, orange juice, brown sugar, cinnamon, nutmeg, and vanilla extract in a small bowl. Fill each apple cavity evenly with this mixture and top with butter cubes.
04 - In a separate bowl, mix oats, flour, brown sugar, salt, cinnamon, and nuts if using. Add cold butter and rub with fingertips until coarse crumbs form.
05 - Spoon oat crumble generously over each filled apple, pressing lightly to adhere.
06 - Pour 1/2 cup water into the bottom of the baking dish to prevent sticking and promote steaming.
07 - Cover the dish loosely with foil and bake for 40 minutes.
08 - Remove foil and continue baking for 25 to 30 minutes until apples are tender and crumble is golden brown.
09 - Allow to cool slightly before serving warm, optionally with vanilla ice cream or Greek yogurt.