For the Apples:

• 4 large baking apples (such as Honeycrisp or Granny Smith)
• 1/2 cup rolled oats
• 1/4 cup chopped walnuts or pecans
• 2 tablespoons brown sugar
• 1/2 teaspoon ground cinnamon
• 1/4 teaspoon ground nutmeg
• 1/8 teaspoon salt
• 2 tablespoons unsalted butter, melted
• 1/2 teaspoon vanilla extract
• 1/3 cup apple juice or water (for baking)

For the Vanilla Custard Sauce:

• 2 cups whole milk
• 4 large egg yolks
• 1/4 cup granulated sugar
• 1 tablespoon cornstarch
• 1 tablespoon unsalted butter
• 1 teaspoon pure vanilla extract
• pinch of salt