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1
Heat the oil in a large pot over medium heat. Add the onion and saute until soft, about 4 minutes. Add the salt, garlic, chili, ginger, lime zest, potatoes, and corn; cook until soft and fragrant, about 3 minutes.
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2
If using, put the corn cobs into to the pot. Add the lemongrass and kaffir lime leaves. Cover all with the vegetable broth and bring to a boil. Reduce to a simmer and partially cover the pot, leaving some space so the steam can escape. Simmer for 45 minutes, until all of the vegetables are soft.
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3
Turn off the heat and remove the lemongrass and kaffir lime leaf from the pot. With an immersion blender, partially blend the soup so it is thick and creamy but with some whole corn kernels left. Alternatively, you can remove about half of the soup and blend it in a standing blender before adding it back to the pot.
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4
Add the coconut milk, cherry tomatoes, Thai basil, cilantro, and lime juice (leaving some of the herbs aside for a garnish - if you like to garnish). Bring the soup back to a gentle simmer for about 10 minutes, until the tomatoes are soft and the soup is hot. Add more salt, to taste.
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5
Ladle the soup into bowls and garnish with the remaining herbs. Top with some sriracha for additional heat and savor the sweet corn (while it lasts).
