Heat oil in a Dutch oven or good-sized saucepan over medium flame. Add onion and garlic. Cook, stirring constantly for about 45 seconds. Add pork belly and red chili flakes, and stir to combine. Cook, stirring occasionally, for 5 to 7 minutes.
Add kimchi, stirring everything to combine, and cook 3 to 5 minutes, stirring occasionally. Add water, broth, soy sauce and red chili paste, stirring to combine. Bring to a boil, reduce heat to a simmer, cover and cook for 5 to 7 minutes, stirring occasionally. Taste the broth and see if you want to dial up the heat a little, either with the chili flakes or paste (sorry, but you can’t dial it down at this point).
Stir in the tofu, cover and cook for about 5 minutes. Taste and adjust seasonings with salt, if needed.
Spoon into bowls, top with scallions and serve. You can serve it with white rice, if you like, or on its own. (You can also let it cool without the scallions, then refrigerate it and reheat it gently the next day for a delicious make-ahead meal.)