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Preheat oven to 375F/190C. Spray a 9" x 13" casserole dish (or similar size) with non-stick spray.
Trim any fat or undesirable parts from three large boneless, skinless chicken breasts and rub chicken on both sides with Poultry Seasoning (affiliate link).
Heat 1 T olive oil in large non-stick pan over medium-high heat and brown the chicken on both sides (just until they are barely brown, about 3 minutes per side; the chicken won't be cooked through at this point.)
Season chicken with a little salt and fresh-ground black pepper as it browns. Remove browned chicken to a cutting board.
While chicken browns, wash mushrooms and dry mushrooms and cut in thick slices. (We cut each mushroom into 3 slices.)
Heat the other 1 T of the oil in the same pan over medium-high heat and cook mushrooms until they're nicely browned and any liquid that's released has evaporated.
While mushrooms cook, cut the chicken into lengthwise strips. It's important to make the slices close to the same thickness, so they cook in the same amount of time. (I could have done better at that!)
Arrange chicken pieces crosswise in the casserole dish and cover chicken with cooked mushrooms.
Turn heat to low in the pan and add chicken stock, balsamic vinegar, Dijon, and Garlic powder.
Whisk it together and then cook 6-8 minutes, or until the mixture reduces and slightly thickens.
Use a rubber scraper to scrape out all the thickened balsamic sauce and drizzle it over the chicken and mushrooms.
Crumble Feta Cheese over the top of the chicken and mushrooms in the dish.
Bake about 20 minutes, or until the thickest part of the chicken tests at least 165F/75C when you test with an Instant Read Meat Thermometer (affiliate link).
If you don't have a thermometer, cut one inside piece of chicken to make sure it's done.
Serve hot, with Cauliflower Rice or Pureed Cauliflower to eat with the flavorful juice that's in the bottom of the dish.