Passion fruit sour soup with tomatoes and tofu

Look at Thai and Chinese sour soup recipes and you'll see there's a whole spectrum of techniques for building acidity into a broth. Today's passion fruit sour soup draws on tart fruit, tomatoes, and lime to achieve that tanginess.

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• Ingredients and variations
• Passionfruit and tomato sour soup with tofu

I'd wager if you asked a random sample of an average Anglo population what they knew of sour soups, many answers would be limited to "hot and sour," but this is a global category of soup. There's the Slavic sourdough soups such as Polish żurek, yoghurt soups like Turkish yayla çorbasi, Thai soups like tom som and tom yum, Vietnamese canh chua, Indian rasams, Ugandan malakwang, sopa de lima from The Yucatán, and countless others I don't remember or will never encounter because the list is so long.

China has its own tradition of sour soups, numerous in style and quantity, with regional variants galore. A discussion of sour soups in China is incomplete without a whiplash-inducing mega-nod toward Guizhou, with its tart fermented rice water and sour tomato soups, both of which have subclassifications that expand the category dramatically. There's also Sichuan suan cai yu, a lacto-fermented vegetable (napa cabbage in the North, mustard greens in the South) soup with a numbing Sichuan peppercorn base.

And finally (but not definitively), we can't ignore the sour soups of Yunnan, prepared with fruits, fermented bamboo, and other acidic ingredients. In my two visits to Dali, in Western Yunnan, I spent many hours using my phone to translate poster displays outside of restaurants. Passion fruit sour beef soup appeared repeatedly in the translations, but a vegan version proved elusive (the one vegan restaurant I found with it on the menu informed me they were no longer serving this dish). I added it to my travel notes to come back to later and moved on.

Ingredients and variations

Well, "later" finally arrived, and with a peak summer garden full of tomatoes as a bonus. I looked to Rednote/Xiaohongshu, the popular Chinese social media platform, for an idea of ingredients and quantities. The basics became clear: fry aromatics as a base; layer with tomatoes and passionfruit; season with soy, vegan oyster sauce, sugar, and salt; then brighten it all up with lime and fresh herbs to serve.

All in all it takes about an hour to prepare. Much of that time is dedicated to readying the tofu, which I boil in salted water and then lightly fry to make it extra sturdy. You can skip this step if you're short on time and add tofu directly to the pot, but if you do then use a firm tofu that can hold up in a liquid dish. You can also just follow the boiling step but skip the frying if you want a softer tofu that still holds up well in a soup.

Vegan oyster sauce is readily available at Chinese grocers. In the UK, Lee Kum Kee is the market leader and they sell two vegan options. First is their newer vegan oyster flavour sauce, which has a distinctive seafood-y taste. Second is their classic mushroom based vegetarian stir fry sauce. Both can be used as a 1:1 for nonvegan oyster sauce in any recipe. It's a handy condiment for stir fries, so it's worth keeping a bottle around.