Preheat the oven to 400°F (200°C).
Slice the Granny Smith apples and the Braeburn or Honeycrisp apple into thin, even pieces.
Place the apple slices in a bowl and drizzle with half of the maple syrup. Mix until coated.
Arrange the apples in a single layer in a small cast-iron skillet.
Bake for 5 minutes.
Remove the skillet from the oven and sprinkle the granola evenly over the apples.
Drizzle the remaining maple syrup over the granola.
Return the skillet to the oven and bake for an additional 10 to 15 minutes, until the apples are tender and the granola is golden.
Remove from the oven and let cool slightly before serving.