Prep Work:
In a bowl, combine the beef broth, chicken bouillon, soy sauce, onion powder, mustard powder, and thyme. Set aside.
In a small container, mix the cornstarch with 3 tablespoons of cold water. Shake to combine and set aside.
Rinse the mushrooms gently and pat them dry. Slice if needed.
Slice each chicken breast lengthwise into 2-3 thinner pieces. If needed, use a meat tenderizer to pound the slices to about ½ inch thick.
Cook the Mushrooms:
Melt butter in a large skillet over medium-high heat. Add the mushrooms and let them brown on each side for 3-4 minutes, undisturbed. Sauté in batches if necessary to avoid overcrowding. Remove the mushrooms from the pan and set aside.
Dredge/Sear the Chicken:
Lightly season both sides of the chicken slices with salt and pepper. Dredge them in flour, shaking off any excess.
Heat olive oil in the skillet over medium-high heat. Add the chicken slices, leaving space between them. Sear for 4-5 minutes on each side until golden brown. Remove the chicken and set aside. Repeat with the remaining pieces.
Deglaze the Pan:
Turn off the heat and remove excess oil, but leave the flavorful brown bits (fond) in the pan.
Add the chicken broth and minced garlic to the pan. Turn the heat back to medium and use a spatula to scrape the bottom of the pan, releasing the fond. Let it simmer until reduced by half, about 4 minutes.
Finish the Sauce:
Add the beef broth mixture to the pan and bring it to a gentle boil. Let it bubble and reduce for about 10 minutes.
Shake the reserved cornstarch mixture and slowly stir it into the sauce. It will thicken quickly.
Lower the heat and stir in the heavy cream. Add the sautéed mushrooms to the sauce.
Return the seared chicken to the pan along with any juices from the plate. Spoon the sauce over the chicken, cover partially, and let it heat through for about 5 minutes.
Serving Suggestion:
Serve the mushroom chicken with mashed potatoes, buttered noodles, and roasted green beans or asparagus for a complete meal.