Lay the slices of lamb in a shallow dish. In a bowl, mix together 2 tablespoons of the soy sauce, 2 tablespoons of the peanut oil, the rice wine/sherry, salt, and Sichuan pepper. Pour the mixture over lamb, and turn to coat. Let marinate for 30 minutes.
In a wok, heat remaining 2 tablespoons of peanut oil until smoking, then add the lamb and garlic. Stir-fry for 2 minutes just until lamb changes color on both sides, then remove from wok and set aside.
Leave 1 tablespoon of oil in wok and pour out excess. Add the broccoli and scallions and stir-fry for 2 minutes. Add the rice vinegar and remaining soy sauce and cook for another 1 minute. Return the lamb to the wok, add sesame oil, and stir-fry for another 1 to 2 minutes, until the lamb, broccoli, and sauce are thoroughly mixed. Serve immediately.