Cook pasta according to package directions. Drain and transfer to a large bowl. While pasta is still hot, stir in sour cream and set aside.
Scald milk by heating in a small saucepan over medium heat until tiny bubbles just appear around the edges.
Add Cheddar and Monterey cheeses, mustard, nutmeg, hot sauce, pepper and salt to scalded milk. Simmer until cheese melts, stirring constantly.
Fold cheese mixture into pasta and transfer mixture to a serving platter.
Combine bread crumbs and Parmesan in a skillet over medium heat. Toast 1 to 2 minutes, until golden brown. Sprinkle mixture over macaroni and cheese and serve.
