Heat 1 tablespoon of the oil in a wok over a high heat until the oil starts to shimmer. Add half the beef, season with salt and stir-fry for 2 minutes. When it begins to colour, lift the beef on to a plate using a slotted spoon. Heat another 1 tablespoon of the oil and stir-fry the rest of the beef in the same way.
Return the wok to the heat and wipe it clean with kitchen paper. Heat the remaining oil and tip in the pepper, sweetcorn, chilli and shallots. Stir-fry for 2 minutes before adding the black bean sauce, the measurement water and the cornflour paste. Bring to the boil, return the beef to the wok and stir-fry until the sauce thickens and coats the stir-fry in a velvety glaze. Serve with rice, if liked.