Preheat the oven to 325°F. Prepare the baking dish (9x13 inch) by greasing it with some butter.
In a medium bowl, add the oats, brown sugar, baking powder, cinnamon, salt, walnuts (optional) and raisins. Mix well to combine.
In another bowl, whisk the eggs. Then add the milk, vanilla and melted butter and whisk again until well combined.
Add wet ingredients to the oat mixture and stir until well combined.
Place the apples evenly on the bottom of the prepared baking dish. Pour the oatmeal mixture over top and spread evenly. Bake for 45-55 minutes, until the top is golden and the oats are set. Serve at room temperature with Greek yogurt, raisins and maple syrup on top.
Notes
Whole grain oats are best in this recipe.
You can sub milk of choice (dairy or non-dairy).
Make sure to use a tart apple like Granny Smith.
Add additional mix-ins like seeds or chips, if desired.