In large nonstick skillet, heat oil over medium heat; cook onion, garlic, salt, pepper and thyme, stirring, for about 5 minutes or until onions are softened. Add mushrooms and increase to high heat, cook, stirring until mushrooms are browned.

Meanwhile, cut chicken into bite-size pieces; stir into skillet. Cook, stirring, for about 4 minutes or until no longer pink inside.Scrape chicken mixture into bowl; set aside.

In small bowl, whisk together stock and cornstarch, pour into skillet. Cook over medium heat, stirring, for about 5 minutes or until thickened.

Return the chicken mixture to the pan.

Increase the heat to medium, stir in the sour cream and mustard and heat through (do not boil!).