Trim any excess fat from meat. Cut meat crosswise into 1/2" slices. Place each slice between two pieces of plastic wrap. Using the heel of your hand, press lightly to 1/8" thickness; remove plastic wrap.
In a shallow dish, combine flour, pepper and salt. Dip meat into flour mixture, turning to coat.
In a large skillet, heat butter and oil over medium-high heat. Add half of the meat; reduce heat to medium. Cook for 2 to 3 minutes or until meat is slightly pink in center and coating is golden brown, turning once. Remove meat, reserving drippings in the skillet. Cover meat and keep warm in 300F oven. Repeat with remaining meat.
Add mushrooms, rosemary and garlic to the reserved drippings. Cook and stir just until mushrooms are tender.
Add chicken broth, scraping up any browned bits from bottom of skillet. Bring to boiling; reduce heat. Simmer gently about 5 minutes or until reduced by half. Stir in mustard, lemon peel and lemon juice; heat through.
Serve the mushroom mixture over meat. If desired, garnish with lemon wedges and additional fresh rosemary.