1: Fire up your oven.
Aim for 220°C on the baking stone inside. Use the thermometer to quickly and easily check the temperature.
2: In a saucepan, add potatoes, milk, cream, butter, and garlic.
Season with salt, pepper and nutmeg, then stir. Bring to a boil over medium heat, then lightly simmer for 8 to 10 minutes. The potatoes should be tender and the sauce slightly thickened. Taste and adjust seasoning as needed.
3: Lightly grease the cast iron pan with butter.
Pour the potato and cream mix into the pan, spreading evenly using the back of a spoon. Sprinkle grated Gruyère and Parmesan over the top.
4: Cook in your Ooni oven for 5 to 6 minutes, turning the pan every 2 minutes, until the sauce is bubbling and the cheese has turned a deep golden brown.
Let cool for a couple of minutes before serving.