In 2-quart saucepan, melt butter over low heat. Stir in flour, 1 teaspoon salt and the nutmeg. Cook over medium heat, stirring constantly, until smooth and bubbly. Gradually beat in milk with whisk. Heat to boiling, stirring constantly; boil and stir 1 minute. Remove from heat; beat in 1/2 cup Parmesan cheese. In small bowl, beat 1/2 cup of the hot mixture into beaten egg. Return egg mixture to pan, beating constantly. Pour over top of pasta.