Preheat oven to 375 degrees F. Grease an 8-inch or 9-inch baking dish; set aside.
In a medium bowl, stir together the oats, half of the walnuts, cinnamon, baking powder and salt; set aside.
In a separate medium bowl, whisk together the milk, maple syrup, egg, half of the melted butter and vanilla extract.
Arrange the sliced apples in a single layer on the bottom of the baking dish. Sprinkle 1 cup of the raisins over top of the apples. Cover the apples and raisins with the oat mixture, using a fork or spoon to spread it into an even layer. Slowly drizzle the milk mixture all over the top of the oats. Rap the baking dish on the counter a couple of time to disperse the milk among the oats. Sprinkle the top with the remaining walnuts and raisins.
Bake for 35 to 45 minutes, or until the top is golden brown and the oatmeal mixture is set. Remove from the oven and let sit for at least 5 minutes. Drizzle the remaining melted butter over top and serve.
Oats: Stick to old-fashioned oats for this recipe. Do not use quick-cooking oats (the texture will be too mushy) or steel-cut oats (they will be under-done).
Walnuts: Toast for extra flavor and crunch (see the tip above). Pecans would be a great substitution; you can omit the nuts entirely if desired.
Maple Syrup: Use pure maple syrup; pancake syrup is too sweet and doesn't have as much maple flavor.
Apples: Substitute your favorite apple if you wish; honeycrisp would be a great alternative!
Raisins: Use regular or golden, or substitute other favorite dried fruit.
Store in the fridge: This recipe will keep fresh in the refrigerator in an airtight container for 4 -5 days.
Store in the freezer: For longer storage, wrap individual portions in plastic wrap and store in a freezer-safe bag for up to 3 months.
To reheat: You can easily reheat individual portions of the oatmeal in the microwave in 30-second bursts until it is heated through. If you want to reheat an entire dish of oatmeal, heat your oven to 350 F/180C and baked, covered with foil, for 15 – 20 minutes.
Variations: See post above for multiple ways to switch up this recipe!