Warm the milk in a saucepan or microwave until just steaming - this prevents lumps in your sauce. Shred the fontina, grate the parmesan, tear the prosciutto, and mince the garlic. Set a large pot of salted water on to boil.

Cook the pasta in well-salted boiling water for 2 minutes less than package directions - it should still have a firm bite in the center. Reserve 1 cup of pasta water, then drain and set aside. The pasta will finish cooking in the oven.

In a large saucepan, melt butter over medium heat. Add the flour and whisk constantly for 2 minutes until the mixture turns golden and smells nutty. Do not let it brown.

Slowly pour in the warm milk while whisking constantly to prevent lumps. Continue cooking and whisking until the sauce thickens enough to coat the back of a spoon, about 5-7 minutes. It should bubble gently but not boil.

Remove from heat. Stir in the fontina and 3/4 cup of parmesan until completely melted and smooth. Add the garlic, black pepper, and nutmeg. The sauce should be thick but pourable - thin with reserved pasta water if needed.

Add the drained pasta, prosciutto, and torn basil to the cheese sauce. Stir until every curve of pasta is coated. Taste and adjust salt - the prosciutto adds plenty, so you may need little or none.

Transfer to a buttered 9x13 inch baking dish. Top with remaining 1/4 cup parmesan. Bake at 375°F for 20-25 minutes until bubbling at the edges and golden on top. Let rest 5 minutes before serving with more fresh basil.