Cook your pasta in salted boiling water according to the package directions. Take it out and set it aside when it’s still al dente since it will cook for a minute or two in the cheese sauce. 

Heat the butter in a saucepan over medium heat. Whisk in the flour until it’s completely incorporated. (This will create a roux that will help to thicken the sauce.) Cook the roux until it’s golden in colour and smells nutty and fragrant. 

Slowly stream in the milk, whisking the entire time. Cook, stirring the milk mixture until it thickens slightly.

Reduce the heat to low. In small batches, add in three quarters of the fontina. Start with a handful at a time, stirring after each addition until the cheese fully melts. Next, stir in the Parmesan, followed by a pinch of salt and pepper, along with the nutmeg and mustard powder.

Place your pasta in the cast iron skillet. Toss the pasta with half of the remaining fontina, reserving the rest for the topping. Pour the cheese sauce over the pasta and toss until fully coated.

Top the pasta with the breadcrumbs and remaining fontina. 

Fire up your Ooni oven. Aim for 170-200˚C (350-400˚F) on the stone baking board inside. (You can check the temperature inside quickly and easily using an infrared thermometer.)

Place the skillet in the center of the oven. Bake for 3 minutes, then rotate the pan and bake for 3 minutes more. 

Bake for an additional 1 to 2 minutes until the top of the mac and cheese is golden brown and crispy. 

Remove from the oven and let cool for 5 minutes before serving.