Heat the stock. Wash and drain the watercress, removing and discarding all but the smallest stalks. Cut off the roots and leaf tips of the spring onions, remove the outermost layer of the bulb and discard.

Slit the spring onions (bulbs and leaves) lengthwise in half, then cut these long, thin pieces into 3/4 in sections.

Place the raw fillet in the freezer for 10 minutes to make it easier to slice into wafer-thin slices. Cut each of the slices in half and then cut into fine strips; place in a bowl and sprinkle with 1 tsp salt and the flour; stir well.

When the stock has come to the boil, stir in the soy sauce, the beef strips and the watercress. Simmer gently for 1 minute only. Add the spring onion pieces and simmer for a further minute, then remove from the heat.

Season with a pinch of freshly ground black pepper and serve.