1. Add the olive oil from the canned anchovies to a large, heavy-bottomed pot or Dutch oven. Reserve anchovy fillets for later.
2. Over medium heat, add the chopped garlic and scallions. Sauté until fragrant, about 1 to 2 minutes.
3. Bring the heat to medium-high. Add the cut-up pork belly to the pot and continue to cook until the meat is no longer pink and slightly browning, for 4 to 5 minutes.
4. Add kimchi, reserved anchovy fillets, gochugaru, and apple cider vinegar. Cook for another 5 minutes until the kimchi starts to look tender.
5. Add onion to the mixture and water and bring it to a boil. Simmer for at least 40 minutes, or longer, or until the pork belly gets super tender and kimchi has absorbed all the flavors.
6. Season it with fish sauce. Then, add cut-up tofu and simmer for 5 more minutes. Add kosher salt if it needs more seasoning.
7. Garnish with scallions, and serve with rice.