Creating Cinnamon Oatmeal Stuffed Apples with Vanilla Custard Sauce involves several straightforward steps:

1. Preheat Oven: Preheat your oven to 350°F (175°C). Prepare a baking dish by lightly greasing it with cooking spray.

2. Prepare Apples: Wash the apples and carefully core them, leaving the bottoms intact. You can use a paring knife or an apple corer for this.

3. Make the Filling: In a mixing bowl, combine the rolled oats, brown sugar, ground cinnamon, and salt. Stir in the melted butter until well mixed. If desired, add chopped nuts and raisins for extra flavor.

4. Stuff the Apples: Generously fill each apple with the oatmeal mixture, pressing it down lightly to pack it in. Place the stuffed apples upright in the prepared baking dish.

5. Add Water: Pour 1 cup of water into the bottom of the baking dish to help steam the apples during baking.

6. Bake: Cover the baking dish with aluminum foil and bake for 25-30 minutes, or until the apples are tender.

7. Prepare Vanilla Custard Sauce: In a saucepan, combine the milk, sugar, cornstarch, and salt. Whisk together and heat on medium until simmering, stirring constantly.

8. Thicken the Custard: In a separate bowl, whisk together the egg yolks and vanilla extract. Slowly pour in the warm milk mixture, whisking continuously. Return the mixture to the saucepan and heat, stirring until thickened.

9. Cool Custard: Once thickened, remove from heat. Let the custard cool slightly before using it as a sauce.

10. Serve: Plate the baked apples and drizzle with warm vanilla custard sauce.