1Mix 2 Tbsp. butter and 2 Tbsp. flour in small pan and cook over medium heat til smooth and thick; don't brown. Set aside.
2Saute onion in 1 Tbsp. butter in large pan over medium high heat til golden brown, stirring constantly. Add water, chicken base and white pepper and mix well. Bring mixture to a boil over high heat, stirring constantly. Reduce heat.
3Mix egg yolk and 1/2 cup flour in medium bowl and whisk til blended. Add milk gradually, whisking constantly til smooth. Whisk the milk mixture into the chicken base mixture and bring to a boil, stirring constantly. Reduce heat
4Add the cheeses and cook til melted, stirring to mix well. Whisk in butter and flour mixture. Cook til thickened to desired consistency, whisking constantly. Add salt and additional pepper if desired. Stir in parsley and seasoning sauce to serve. Serves 6