Cut the kimchi into bite-size pieces and reserve 120 ml / 1/2 cup of juice from the container.
Fry the pork belly in a dry heavy pot over medium-high heat until the edges brown and fat renders, about 4 minutes.
Add the kimchi, onion, garlic, gochugaru, and sugar. Stir-fry for 5 minutes, until the kimchi darkens and smells toasty rather than raw.
Stir in the gochujang and guk-ganjang and cook for 30 seconds.
Pour in the kimchi juice and broth. The liquid should sit about 2 cm below the top of the solids.
Bring to a boil, then simmer uncovered for 10 minutes so the broth reduces and concentrates.
Lay the tofu slices on top and simmer 3 minutes more without stirring.
Scatter the scallions, take the pot off the heat, add the sesame oil, and serve straight from the pot with rice.