Melt butter in a 8-quart stock pot over medium heat. Add onion and sugar, and cook, stirring constantly, 20 to 25 minutes or until browned.
Stir in garlic, and cook 1 minute. Stir in flour, and cook, stirring constantly, 1 minute. Gradually add milk and whipping cream, and cook, stirring constantly, until mixture is slightly thickened. Stir in salt and pepper.
Whisk together egg yolks in a small bowl until blended. Gradually add 3 tablespoons hot mixture, whisking constantly. Stir yolk mixture into remaining hot mixture. Stir in chopped fresh parsley.