Yuzu is one of my favourite citrus flavours; I like it so much that I scan dessert menus (especially those in Japanese restaurants) for it. I even have a yuzu tree - imported by a friend at great expense - on my balcony. It hasn't borne fruit yet, but hope springs eternal and I dream of the day when I can reach through the leaves, carefully avoiding the tree's many sharp spikes, and pluck a home-grown yuzu.
The squat, round yellow fruit has bumpy skin - it isn't the prettiest type of citrus but the flavour and aroma are intense and distinctive. The flesh is quite seedy and not very juicy, but it's the thick skin that's primarily used, as a flavouring. Japanese chefs grate the zest at the last minute over raw fish and salads, use it to flavour miso, simmer the peel with other ingredients in stews, and use it in desserts. It's one of the ingredients in the peppery, spicy condiment yuzu kosho, which is so strong that most chefs use just a small dab to flavour a dish. In Korean shops, you can buy jars of yuzu marmalade, which is delicious as a type of tea if you stir a spoonful of it into boiling water.
Fresh yuzu fruit isn't easy to find in Hong Kong - Japanese supermarkets such as Apita (in Taikoo Shing), Fresh Market (in Sogo, Causeway Bay) and City'super sell it intermittently. It's much easier to find packaged yuzu products: it's used in ponzu dipping sauce; the zest is used to flavour salt and sugar, and also dried and ground to a powder; and the skin is freeze-dried (it needs to be rehydrated).