Grease the slow cooker insert lightly with cooking spray or butter. Peel and core apples, then chop into ½-inch pieces so they soften evenly, merging their sweetness overnight.

Add oats, milk, water, and maple syrup for the oatmeal base, mixing to ensure a creamy, flavorful consistency.

Sprinkle ground cinnamon and optional nutmeg over the mixture. Stir in vanilla extract.

Set the slow cooker to low and cook for 7–8 hours, or until oats are tender and creamy.

Gently stir the oatmeal before serving to incorporate any settled milk, ensuring a velvety texture.

Scoop into bowls and garnish with chopped nuts, fresh apple slices, or a dollop of yogurt.