Heat vegetable oil in a nonstick skillet.

In a medium bowl, mix 2 tablespoons of vinegar, the mustard and garlic. Add chicken and turn to coat.

Transfer chicken and marinade to skillet. Saute chicken until cooked through, about 3 minutes on each side. Transfer chicken to a platter and keep warm.

Heat remaining tablespoon of oil in skillet. Saute mushrooms for 1 minute. Add broth, thyme and remaining tablespoon of vinegar. Cook, stirring occasionally, until mushrooms are deep brown about 2 minutes longer.

Serve chicken topped with mushrooms.