Egg and watercress sandwiches were mentioned frequently in the British period novels I read as a child. The watercress, served on crustless bread with tea and polite conversation, was made to sound exotic and expensive, which I found puzzling. In my family, my grandmother would use large quantities of watercress to make tonic soup; we would drink the broth and throw away the vegetable because after being simmered for such a long time, there was no nutritional value left in it. Watercress is in season in the spring. The variety we eat in Hong Kong is milder than English watercress, which has a peppery bite. For all varieties, look for small green leaves without any flowers or yellow or brown tips - these are indications the vegetable is old, which means the flavour will be bitter. The stalks should be bushy with lots of leaves, rather than long and leggy. The vegetable, which grows in water, should be rinsed thoroughly before use and the thick, tough stems trimmed off and discarded (or they can be used for vegetable stock). In traditional Chinese medicine, the vegetable is believed to be cooling, making it good to consume in limited quantities when it's hot and humid. Blanch some meaty pork ribs in boiling water then drain and rinse them. Put the bones back in the pan, add water, a few slices of fresh ginger, a couple of spring onions, a dried and salted duck gizzard and some Chinese dried dates. Bring to the boil, add a large bunch of watercress then lower the heat and simmer for about two hours. The soup will take on a khaki-green hue. Season the broth with salt and white pepper. The ribs can be eaten (dip them in soy sauce) but the watercress will be stringy and flavourless. The creamy version of watercress soup tends to be a lot prettier than the Chinese soup, with a fresh, pale green colour. Cook a chopped leek in butter over low heat. Add some diced, cubed potatoes and stir to coat with the fat then add good-quality chicken broth. Bring to the boil then lower the heat and cook until the potatoes are tender. Add finely chopped watercress (leaves and the thin stems only) and cook until tender. Puree the soup in a blender or food processor and pour back into the saucepan through a fine sieve. Add some cream and salt and pepper to taste then bring to the simmer again. Serve with croutons sauteed until crisp in clarified butter and garnish with a few watercress leaves.