Preheat the oven to 350-degrees.
In a small bowl, combine the dry ingredients. Then, using a fork, cut in the butter.
Core the apple and peel the top third or so. You can use a melon baller to make the opening in the apple a little larger if you like.
Fill the hole with the oat mixture, pressing it down into the core.
You'll have more filling than what can fit in the apple, and that's OK. Mound the extra up on the top.
Place the apple in the skillet.
Pour the liquid in the bottom of the iron skillet.
Cover with foil and bake for 30 minutes. Remove the foil and baste with the liquid every 15 minutes.
Finish baking until the apple is tender.