Heat the peanut oil in a medium pot over medium-high heat. Set aside a couple tablespoons of scallions for a garnish. Add the remaining scallions, garlic, and ginger to the pot. Cook, stirring, until fragrant, about 1 minute. Add the choi and potato. Pour in 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. Transfer the soup to a food processor or a blender and purée. Ladle soup into individual bowls.
Garnish each bowl with a dollop of sour cream and some chopped scallion. Serve immediately.