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Swirl the oil into a large pot and heat over medium heat until it shimmers. Add the garlic and onion and stir and cook until the onion turns translucent, 2 to 3 minutes.
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Add the chopped rhubarb and stir and cook for 1 to 2 minutes, until the rhubarb pieces soften and turn a shade paler. Pour in the vegetable stock and bring to a boil.
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Add the white cabbage stalks and cook for 1 to 2 minutes. Add the cabbage leaves and the tomatoes, and cook for another 1 to 2 minutes until the tomatoes soften a little and the cabbage is cooked but still crisp (or for however long you may like).
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Stir in the sugar and the fish sauce. Taste and adjust the flavors as needed. Turn off the heat and add the bean sprouts, they will cook in the residual heat but still be crunchy. Sprinkle with cilantro and green onions with or without abandon.
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Serve immediately on the table family style, or in individual bowls with steamed rice and another dish or two. Or just have a huge bowl of it with rice. You can also have lemon wedges on the table if you’d like to spritz some juice on your soup.