Preheat your oven to 350°F (175°C) to prepare for the baking process.
Using an apple corer or a sharp knife, carefully core each Granny Smith apple, ensuring that you leave the bottom intact to create a pocket for the filling.
In a medium mixing bowl, combine the rolled oats, brown sugar, ground cinnamon, chopped walnuts or pecans, raisins or dried cranberries, melted butter, and a pinch of salt. Stir the ingredients together until they are well blended and form a cohesive filling.
Take spoonfuls of the oat mixture and gently fill each cored apple, packing the filling firmly inside to ensure they are generously stuffed.
Arrange the stuffed apples upright in a baking dish. Pour apple juice or water into the bottom of the dish, which will help create steam during the baking process, ensuring the apples remain moist.
Cover the baking dish with aluminum foil to keep in moisture and bake in the preheated oven for 25 minutes. After this initial baking time, remove the foil and continue baking for an additional 15-20 minutes, or until the apples are tender and the filling has turned a beautiful golden brown.
Once finished baking, allow the apples to cool for a few minutes before serving to avoid burning your mouth on the hot filling.
Notes
Serve warm with ice cream or whipped cream for an indulgent treat.