1. Cut thickest part of beef across the grain into 1 1/2-inch-long pieces. Cut thinnest end into 2-inch-long pieces. Turn each piece onto a cut side and press down firmly to flatten.
2. Sprinkle meat evenly on each side with pepper, pressing it onto the surface.
3. In an 11- to 12-inch nonstick frying pan, combine shallots and 1 tablespoon butter. Stir over high heat until shallots are limp, about 1 1/2 minutes. Add white wine, vinegar, broth, dry mustard, Dijon mustard, and cream. Boil over high heat, stirring, until reduced by half (to 3/4 cup), 5 to 7 minutes. Pour sauce into a small bowl and set aside. Rinse pan and wipe dry.
4. Return frying pan to high heat. When hot, add remaining butter and swirl until it melts, then add meat (pieces should all fit in pan without overlapping). Brown beef on one side, about 2 1/2 minutes. Turn over and continue to cook until brown, about 2 1/2 minutes more for rare.
5. Transfer beef to a warm plate. Add sherry, brandy, and sauce to pan. Stir until bubbling. Pour the sauce, and any juices drained from the meat, onto plates. Top with the beef. Add salt to taste.