Pour in the beef broth mixture, increasing the heat slightly to bring it to a gentle boil. Let the sauce reduce for about 10 minutes.
Shake the cornstarch mixture again and slowly pour it into the sauce, stirring constantly until the sauce thickens. Lower the heat.
Stir in the heavy cream, then add the cooked mushrooms.
Return the chicken to the pan, along with any accumulated juices. Spoon the sauce over the chicken, partially cover, and let it warm through for about 5 minutes.
Serve with mashed potatoes, buttered noodles, or roasted green beans for a complete meal.