In a small bowl, combine the tamarind, soy sauce, sugar, and lemon juice. Stir until the sugar is dissolved. Add the garlic and chiles.
Put the beef in a shallow dish, pour the marinade over it, and marinate in the refrigerator for about 30 minutes, turning several times.
Take the meat from the marinade and drain it well so it’s not dripping wet; reserve the marinade. Heat a wok or large heavy skillet until very hot. Add a small amount of oil and swirl the pan to coat. Add about half of the meat and cook over high heat, tossing and stirring constantly, just until the meat is no longer pink, about 1 minute. Transfer it to a bowl and keep it warm. Repeat with the rest of the meat, again transferring it to the bowl.
When all the meat is cooked and removed from the pan, add a bit more oil to the pan. When it’s hot, add the bok choy and broccoli. Stir-fry the vegetables until hot and slightly wilted, about 3 minutes. Transfer to the bowl with the meat. Pour the marinade into the wok and bring it to a boil over high heat. Cook until slightly thickened and glossy. Return the meat and vegetables to the wok and stir gently to coat with sauce. Serve immediately, with rice if you like.