Begin by transferring the pumpkin pie filling into a container suitable for freezing. Place it in the freezer for a few hours until it becomes solid.
When ready to prepare the smoothie, add the whole milk and yogurt to a blender. Break the frozen pumpkin pie filling into chunks, then add to the blender. Blend until the mixture becomes smooth and the pumpkin filling is thoroughly crushed. Adjust the texture by adding more milk or yogurt if a thinner consistency is preferred.
Pour the smoothie into glasses, sprinkling oat flakes on top for a delightful finish. Serve immediately for the best taste.