Add all ingredients to the prepared baking dish - oats, apples, maple syrup baking powder, cinnamon, nutmeg, salt, milk, egg, melted butter, and vanilla extract. Stir together until well combined.
2 1/4 cups old-fashioned rolled oats, 1 apple, 1/4 cup maple syrup, 1 teaspoon baking powder, 1/2 teaspoon cinnamon, 1/2 teaspoon nutmeg, 1/2 teaspoon salt, 2 cups milk, 1 large egg, 1 teaspoon vanilla, 2 Tablespoons unsalted butter