Melt butter or margarine in a saucepan over medium heat,saute onion and garlic,Add tomatoes. Add heavy cream, stirring constantly. Stir in salt, nutmeg, grated Parmesan cheese, and grated Romano cheese. Stir constantly until melted, then mix in egg yolk. Simmer over medium low heat for 3 to 5 minutes. Garnish with additional grated Parmesan cheese, if desired.