Move slider to AIR FRY/STOVE TOP position. Select SEAR/SAUTÈ and set to 4. Select START/STOP to begin. Add butter to pot and preheat for 2 minutes.
Stir in onion and lardons fry for a few minutes to brown, remove from pot. Turn off unit. Allow to cool for a few minutes.
Layer potatoes into pot with onion, lardons, garlic and season to taste.
Pour over stock and top with a layer of grated Gruyère.
Close the lid and move slider to middle position. Select STEAM AIR FRY and set temperature to 180 ̊C. Set time to 20 minutes. Press START/STOP to begin cooking (this will STEAM for approximately 11 minutes before switching to STEAM AIR FRY).
When cooking is complete, remove pot from unit and serve gratin hot.