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Heat the peanut oil in a medium pot over medium high heat. Set aside a couple of tablespoons of scallions for a garnish. Add the remaining scallions, garlic and ginger to the pot. Cook, stirring, until fragrant, about 1 minute.
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Add the bok choy and potato. Pour the stock or water and add the salt, pepper and hot pepper flakes to taste. Increase the heat and bring to a boil; cover, reduce heat and simmer until the potato is tender, about 20 minutes. Remove the pot from heat. Stir in the toasted sesame oil.
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Transfer the soup to a food processor or a blender and puree. Serve immediately, garnished with a dollop of sour cream and the remaining scallions.