Bring a large pot of water to a boil and add enough salt to make water taste salty. Add macaroni and cook according to package directions. Drain pasta in a colander and return to pot. Cover and cook over low heat until all water has evaporated, 30 seconds to 1 minute. Add milk, salt, pepper, butter, nutmeg, and cheeses and stir until all the cheese has melted. If necessary, turn the burner to a very low heat to melt remaining cheese.

Serve immediately, garnished with more Parmesan cheese if desired.