In a pan, melt the butter over low heat.

Add the onion, garlic, and mushrooms (if using) and saute until soft.

Turn up the heat and add chicken to the mixture. Cook the chicken until it's lightly browned on both sides.

Add chicken broth, stirring to mix all flavors, and cook until chicken is done through.

Add the milk before mixing in the arrowroot starch (or flour) and water.

Stir constantly and cook over medium heat until the mixture is thickened. Serve.