Preheat oven to 375 degrees F. If using cast iron, preheat pan in oven. If using 9x13 pan, grease with cooking spray.
In a large bowl, combine oats, walnuts, baking powder, cinnamon, salt, and brown sugar. Mix well.
Add the coconut oil or butter and mix until well coated.
In a separate bowl, whisk together eggs, milk, and vanilla extract.
Now combine the egg mixture with the oat mixture. Mix until well incorporated.
Pour into pan.
Bake for 30 to 40 minutes, or until done. Enjoy!