Trim the steak of all fat and then cut the meat into thin slices across the grain. Heat half the oil in a wok or large frying pan, add the beef, in 2 batches, and cook, stirring, until well browned all over. Transfer to a bowl.
Heat the remaining oil in the pan, add the onion and mangetout and stir-fry for 2 minutes. Add the garlic, ginger and chilli and stir-fry for 1 minute. Add the black bean sauce and cook, stirring, for 5 minutes or until the sauce begins to thicken. Season to taste with salt and pepper and serve immediately with steamed rice or egg-fried rice.