1. Prepare the potatoes: Wash and peel your potatoes. Slice them into thin rounds, aiming for a thickness of about 1/8 inch. This ensures they cook evenly and absorb the cheese sauce beautifully.

2. Make the cheese sauce: In a saucepan on medium heat, melt butter and whisk in flour to form a roux. Slowly pour in milk, stirring until the mixture thickens. Add minced garlic, salt, pepper, and your preferred seasonings. Gradually mix in grated cheddar and Gruyère (or Colby Jack) until the sauce is smooth and velvety. Adjust with more milk if needed for the perfect consistency.

3. Layer the potatoes and cheese sauce: In an air fryer safe dish, start with a layer of potato slices. Pour some cheese sauce over the potatoes, ensuring even coverage. Repeat the layers until all potatoes and sauce are used, finishing with a generous layer of cheese on top. This layering technique is crucial for that perfect au gratin texture.

4. Prep the air fryer: Preheat your air fryer to 360°F. This cooking temperature is ideal for cooking the potatoes thoroughly while achieving a beautifully golden top.

5. Air fry to perfection: Place the dish in the air fryer basket. Cook for about 20 minutes, or until the potatoes are tender and the top is golden and bubbly. Depending on your air fryer model, cooking times may vary slightly, so keep an eye on it.

6. Rest before serving: Once done, let the au gratin potatoes rest for 5 minutes. This allows the sauce to thicken slightly and makes serving easier.