This recipe is the perfect reason to splurge on imported aged Provolone and Parmigiano Reggiano since they will add all the flavor you will want in this four cheese sauce.
You can vary the cheeses for this sauce, but do not replace Gorgonzola with Danish Blue as it will be too salty. Definitely use four cheese pasta sauce when you’re making Macaroni and Cheese because four cheeses are better than one.
Heat the broth (you can use chicken or vegetable broth instead of Parmesan Broth) until it is simmering. In a heavy sauce pan melt the butter until foamy. Stir in the flour and cook, stirring all the time until the mixture is bubbling but not browning. Add the broth and stir with a whisk to remove lumps. Once the mixture is smooth, add the milk.
Add the mustard powder, pepper and nutmeg. Cook the sauce on very low heat for about 5 minutes, stirring frequently to prevent sticking.
Add the Provolone, Fontina and Gorgonzola and stir to melt. Add the Parmigiano Reggiano last, and taste to see if the sauce needs any salt. This will depend on the saltiness of the cheeses.
If you are making this sauce in advance, put a piece of parchment paper on top of the hot sauce to prevent a skin from forming. The sauce will thicken as it cools.
When you reheat this sauce, stir continuously with a spatula to prevent burning.