Cut pork into four 4-oz pieces. Between sheets of plastic wrap or waxed paper, flatten pork pieces with meat mallet to 4x3 1/2 inches in diameter, less than 1/2 inch thick.
Sprinkle both sides of pork medallions with mixed-pepper seasoning and salt; press in seasonings. In 12-inch nonstick skillet, heat oil over medium-high heat. Add pork; cook about 5 minutes, turning once, until browned.
Meanwhile, in small bowl, mix broth and flour until smooth. Add broth mixture and remaining ingredients to skillet. Heat to boiling; reduce heat. Cover and cook 8 to 10 minutes, stirring occasionally, until pork is no longer pink in center and mushrooms are tender.