Meanwhile, in 12-inch skillet, heat butter over medium heat. Add pancetta; cook about 4 minutes or until lightly browned. Add beef; cook 5 to 7 minutes, breaking as many lumps as possible, until no longer pink. Add garlic and flour; cook and stir 2 minutes. Add milk; stir to combine. Cook and stir until mixture comes to a boil. Remove from heat; stir in 1/2 cup of the Parmigiano-Reggiano, the mozzarella, salt, pepper and nutmeg. Remove from heat; cool 5 minutes. Stir in egg.