Marinate the Pork: Combine the pork pieces in a bowl and season them generously with salt. Add the minced garlic, grated ginger, soy sauce, sesame oil, and fish sauce. Toss everything together until the pork is well coated. Allow it to marinate for at least 20 minutes at room temperature or cover and refrigerate overnight for deeper flavor.
Prepare the Bok Choy: Slice the baby bok choy into two parts—separating the stems from the leaves. Keep these parts distinct as they will be added to the soup at different stages of cooking.
Cook the Pork and Aromatics: Heat a sturdy soup pot over high heat and add the peanut oil. Once the oil is hot, add the marinated pork and sear it until browned, about 5 minutes. Lower the heat to medium, add the chopped onion, and cook while stirring until the onion softens, roughly 5 minutes. Mix in the gochujang and gochugaru, cooking for 1 to 2 minutes until the paste turns a deeper red color and its aroma is released.
Simmer the Soup: Add the chopped kimchi along with its juices, the bok choy stems, the stock, 4 cups of water, and a pinch of salt to the pot. Bring everything to a rolling boil, then reduce the heat to maintain a steady simmer. Let the soup cook for about 20 minutes, stirring occasionally. Taste the broth and adjust the seasoning if needed.
Add Finishing Ingredients: Shortly before serving, gently stir in the bok choy leaves and the tofu cubes. Allow the tofu to warm through and the bok choy leaves to wilt. Once these ingredients are tender and combined, remove the soup from the heat.
Serve and Garnish: Ladle the soup into individual bowls. Garnish each serving with a sprinkling of scallions and fresh cilantro leaves. Serve the soup hot and enjoy.