Stir in the whole egg and 3 yolks, and then stir in the milk. (foodnetwork.com)
Gradually stir in the milk until the mixture is smooth and creamy. (modernmom.com)
Slowly add broth while stirring with a whisk to evenly incorporate, then slowly stir in milk substitute while continuing to stir until mixture is smooth and cooked to desired thickness. (angelaskitchen.com)