Heat the sesame oil in a large soup pot over medium-high heat. Add the pork shoulder, season with salt and pepper, and cook, stirring often, until browned and crispy, about 10 minutes.
Add the chopped onion, kimchi, kimchi juice, fish sauce and soy sauce and stir to combine. Add enough water to nearly cover the pork mixture (about 4 cups), cover, and bring to a boil. Immediately reduce the heat and simmer until the onion and kimchi are softened and the soup is thick, about 20 minutes.
Stir in the greens, tofu and scallions. Cover the pot and simmer for about 10 minutes until the greens have wilted and softened and to combine the flavors. Taste and season with more soy sauce if needed. Serve with white or brown rice.