Preheat oven to 400 degrees F (200 degrees C).

Line a small baking sheet with parchment paper.

In a small bowl, whisk together brown sugar, cinnamon, nutmeg, and cardamom.

Add apple slices to the bowl and toss to coat with the spice mixture.

Spread the coated apple slices onto the prepared baking sheet.

Place the oats on a separate small baking sheet.

Place both baking sheets in the preheated oven.

Remove the oats from the oven after 5 minutes and set aside to cool.

Allow the apples to bake for another 5 minutes, for a total of 10 minutes.

Remove the apples from the oven and let them cool for about ten minutes.

In a blender, pulse the cooled oats until finely ground.

Add the roasted apples, apple juice, and vanilla extract to the blender.

Blend until the mixture is smooth and creamy.

Pour the smoothie into a glass and serve immediately or chill for later consumption.