INSTRUCTIONS
[Make the Sauce]
1. In a small saucepan over medium heat, warm the milk until small bubbles begin to form around the edge of the pan. Remove from the heat.
2. In a large saucepan over low heat, melt the butter. Add the flour a little at a time, whisking to incorporate. Raise the heat to medium-low and very gradually whisk in the hot milk. Cook, stirring frequently, until the mixture is thick and creamy, about 15 minutes. Add the Gruyère, fontina and cayenne and season with salt. Cook, stirring frequently, until the cheeses are melted, about 2 minutes. Stir in the Parmigiano-Reggiano and set the sauce aside.
[Cook the Pasta]