Herbs and health bolstering or even "medicinal" ingredients are not uncommon additions. Fancy a little oviductus ranae in your dessert soup? How about a nice big bowl of turtle shell jelly? It sounds a little unsettling, but the outcome is frequently delicious and you may be pleasantly surprised to find yourself asking for seconds and even thirds. That's the official party line anyway ;) Personally, if I can't tell what it is, I'll pass, thank you very much! Great thing about sweet potatoes is, you can tell one, a mile away. Now you know why this is my favourite tong sui.
You don't need many ingredients for sweet potato soup, and the classic version features only sweet potato slices in a ginger scented syrup sweetened with rock sugar. This version is a local favourite, embellished with pandan, dried longans and rich, fudgy palm sugar, all of which add up to an intensely flavourful and headily perfumed bowl of healthy, delicious goodness.... without the funky amphibious or reptilian bits and bobs.
Ginger, a proven and trusted digestive aid, is a must as sweet potatoes are believed to be 'windy'. Eating more than a few slices, may cause a gassy tummy and extremely uncomfortable bloating. Trust me, you want ginger in this! Pandan leaves are a wonderful and frequent addition, but can be replaced with vanilla or simply omitted. If longans are not your thing, or difficult to find, dried Chinese dates are a great stand in. If you're working around a bare-bones pantry set up, do it the old fashioned way, with just sweet potatoes, ginger and whatever sugar you have, or like.
This is a very simple, undemanding dish to prepare, but, the touchstone for a well made bowl is the clarity of the syrup. The Chinese refer to this as "cheng", meaning clear. The sweet potatoes must be perfectly tender, without any hint of mushiness, so they don't disintegrate or crumble into the syrup, thus clouding it. The longans must be cooked until rehydrated to the core, but not simmered so long, that their essence is leached out into the syrup, thus rendering them tasteless. Above all, the syrup that carries both ingredients must be flavourful, fragrant, subtly sweet so as not to overpower the flavour of the sweet potatoes and longans, and crystal clear so as not to visually obscure them.
The best way to achieve this ideal, is to cook the sweet potatoes separately from the other ingredients. This may seem like a lot of hoo-ha for what amounts to tarted up boiled sweet potatoes, but once you have taken it in with your eyes, your nose and finally, your tastebuds, you will understand why sometimes, it pays to be just a little bit... persnickety. Cantonese cuisine, at its heart, is uncomplicated cooking, showcasing the best of a few good ingredients, but, technique is everything. And, it may be simple, but damn, if it ain't pretty!