Wrap the beef in clingfilm and place in the freezer for 1–2 hours until it has become quite firm. Meanwhile, if using, soak the pieces of citrus peel for the marinade in hot water for about 30 minutes until softened, then drain and chop finely.
Remove the beef from the freezer, unwrap it and slice it into thin strips against the grain. Put the strips in a non-metallic dish. Whisk together all the marinade ingredients, including the grated orange rind, if using, pour over the beef and stir to coat thoroughly. Set aside to marinate at room temperature for about 30 minutes, or until the beef is completely thawed.
Preheat a wok or large, heavy-based frying pan. Add 1 tablespoon of the oil, swirl it around the pan and heat until hot. Add about half the beef and stir-fry over a high heat for 3 minutes. Transfer the beef to a plate using a slotted spoon. Add 1 further tablespoon of oil to the pan and stir-fry the remaining beef in the same way. Transfer to the plate using a slotted spoon.
Heat the remaining oil in the wok or pan, then add the shallots, stock, soy sauce, rice wine or sherry and any juice from the tangerines. Sprinkle in the chilli, sugar to taste and a little salt and pepper. Bring to the boil, stirring constantly, then cook for about 5 minutes, until the liquid has reduced.
Return the beef to the pan and toss for 1–2 minutes until all the ingredients are coated with the sauce. Add about two-thirds of the tangerine segments and toss quickly to mix, then taste for seasoning. Serve hot, garnished with the remaining tangerine and coriander.