Trim fat from meat. Cut meat crosswise into 1/2-inch-thick slices. Place each slice between two pieces of plastic wrap. With the heel of your hand, press each slice till about 1/8 inch thick. Remove plastic wrap.
In a shallow dish combine flour, pepper and salt. Coat meat with seasoned flour shaking off excess.
In a large skillet heat butter and oil over medium-high heat. Add half of the meat and cook for 3 to 4 minutes or till golden brown on outside and slightly pink in center, turning once. Remove from skillet, reserving drippings in skillet. Keep warm while cooking remaining meat.
Reduce heat to medium. Add mushrooms, snipped fresh or dried rosemary and garlic to drippings in skillet. Cook and stir just till mushrooms are tender. Add broth, scraping up any browned bits on bottom. Bring to boiling. Boil gently about 5 minutes or till reduced by half. Stir in mustard, lemon peel and lemon juice. Heat through.
Spoon mushroom mixture over meat. If desired, garnish with lemon wedges and fresh rosemary sprigs.