Preheat oven to 425°F. Melt 1 stick butter in medium saucepan over medium heat. Add flour and whisk until smooth and mixture starts to bubble. (About 2 minutes) While stirring, gradually add the milk and continue whisking until the sauce is creamy.
Simmer the sauce for about 10 minutes until it thickens and you can coat the back of the spoon. Remove from the heat and add the prosciutto, 1 cup fontina, nutmeg and pepper. I've found this sauce does not need salt as you get plenty of salt from the prosciutto and cheese.
While sauce is cooking bring to boil a large pot of water. Boil the rigatoni for about 7 minutes or just 2 minutes less than the instructions say. You don't need to cook all the way as it will cook the rest of the way in the oven.
Grease a 13x9 inch baking dish and add the drained pasta and sauce. Mix thoroughly, making sure all the pasta is coated. Top with the rest of the fontina cheese, Parmesan and 3 tablespoon of diced butter. Bake for 25 to 30 minutes or until the top is golden brown and bubbling. Let rest for 5 minutes before serving.
Notes
No overcooked pasta – cook the rigatoni al dente according to the package instructions. The pasta will finish cooking in the oven while it’s in the sauce.
Freshly grated cheese – don’t use pre-shredded cheese. It doesn’t taste as good and also has additional ingredients that you don’t need.
Stir constantly – once the milk is added, you must stir constantly as it heats up and becomes thick. Otherwise you’ll have burnt milk on the bottom.
Lots of sauce – who likes a dry baked pasta? At first when making this, you’ll think it is too much sauce but believe me when that the rigatoni soaks it up.