Preheat oven to 400F.
Freshly grate all the cheeses. Reserve 1 ½ cups for the topping.
Bring a large pot of water to a boil. Salt generously, then add the pasta to cook until al dente. Drain and set aside.
Add a large, oven-safe skillet over medium heat.
Add the butter to the skillet to melt. Once melted, add the garlic to sauté 30 seconds, then stir in the flour until fully absorbed by the butter.
While whisking, gradually pour in the milk. Add about ½ cup at a time, then whisk until mixture thickens, about 30 seconds, before adding more.
Gradually pour in the heavy cream as you whisk.
Once slightly thickened, stir in ½ cup of the freshly grated cheeses at a time.
Stir in the salt, black pepper, and nutmeg.
Stir in the pasta until well combined. Add the first pound of cooked pasta, then add more if desired.
Evenly distribute the remaining grated cheeses that you set aside on top.
Bake for 20-25 minutes, until cheeses are bubbly and the top is beginning to brown.
Broil the last 30 seconds to 1 minute for a deeply golden brown top.
Let rest at least 10 minutes before enjoying.
