Put pork belly in a bowl. Add garlic, ginger, soy sauce, sesame oil and fish sauce. Toss well to coat and let marinate for 10 minutes.
Set a heavy-bottomed soup pot over medium heat. Melt butter, then add pork belly mixture and let it cook gently for 5 minutes. Add onion and cook, stirring, until softened, about 5 minutes. Turn heat to medium high and add kimchi, gochujang and gochugaru. Let mixture simmer for 2 minutes.
Add kimchi juice and water (or broth, if using) and bring to a boil. Reduce heat to a brisk simmer and cook for 20 minutes. Taste broth and adjust seasoning.
Just before serving, add tofu and stir gently to combine. When tofu is heated through, ladle into bowls and garnish with scallions.