In the newly washed pot, fill the bottom with old kimchi + juice. Nestle the pork belly on top. Bring to a boil and simmer, covered, until the meat is tender and cooked through. Depending on thickness, this should take 20-30 minutes.
The pork belly should be tender enough to slice easily. After 20 minutes, I test by lightly tapping with my tongs. There should be no "bounce" to the meat. Instead, the pork belly should have some give and feel soft.
Using tongs, remove pork belly from the pan and transfer to a cutting board. Slice into 1/2-inch slices. Return pork belly to the pan, keeping the slices shingled neatly.
Slice tofu into 1/2 inch slices and shingle slices in between the pork belly. There should be alternating rows of shingled pork belly and shingled tofu slices.
Cover and simmer until tofu is heated through, about 5 minutes. Uncover and spoon kimchi juices over the tofu. Drizzle with sesame oil and garnish with green onion and sesame seeds. Serve with rice and other banchan (side dishes). Enjoy!