Tarragon Chicken

3 oz. cremini mushrooms 1 pnd. Chicken breast 1 small onion or tsp dried onion 1/2 c. chicken broth  or 1 cup water&3 boullion cubes 2 TBL Dijon mustard ¾ tsp. dried tarragon ½ tsp salt ¼ tsp pepper 2 c. cream 2  TBL cornstarch 2 c. cooked rice Slice mushrooms, place in saucepan along with chicken and onion. Whisk together broth, mustard, tarragon, salt and pepper and pour over chicken. Cook. When done blend cream and cornstarch and pour into crockpot to thicken. Serve over rice.