Preheat oven to 350 degrees Fahrenheit and line a baking sheet or small Pyrex dish with parchment paper.
Slice the apple in half and scoop out the seeds with a spoon. Place the apple halves cut-side up on the baking sheet.
In a small bowl, mix the oats, cinnamon, brown sugar, nutmeg and nuts together.
Fill the centre and top off the apple with the oat and nut mixture.
Crumble the cold butter over the oat and nut mixture.
Bake for 35-40 minutes. Allow to cool for 5 minutes before enjoying!