Melt the butter in a medium saucepan over medium heat, add the garlic and cook for 30 seconds. Add the onions and cook until softened, about 4 minutes.
Add the prepared kimchi and juice and 3 cups of water. Bring to a boil, reduce the heat and simmer for 10 minutes.
Meanwhile, drain the tuna, breaking it into chunks. Add the tuna, tofu, fish sauce, sesame oil and soy sauce and simmer until heated through, for 3 minutes.
Serve over rice and top with scallions, nori and sesame seeds.