Prep the aromatics and protein. Slice the onion and green onions.
Cut the tofu into cubes. If using pork belly, slice it thin so it renders and browns quickly. Have the kimchi roughly chopped into bite-size pieces.
Heat the pot. Set a medium pot over medium heat and add the neutral oil.
When it shimmers, add the pork belly (if using) and cook 3–4 minutes until lightly browned and some fat has rendered. If skipping meat, warm the oil and go straight to the next step.
Sauté the kimchi and onions. Add the onion, chopped kimchi, and the white parts of the green onions. Cook 4–5 minutes, stirring, until the kimchi softens and turns glossy.
This step builds flavor, so don’t rush it.
Bloom the seasonings. Stir in the garlic, gochujang, and gochugaru. Cook 30–45 seconds until fragrant. This helps the chili and paste release color and depth.
Deglaze and add broth. Pour in the kimchi juice, soy sauce, sugar (if using), and 3 cups of broth.
Scrape up any browned bits from the bottom. Bring to a gentle boil.
Simmer. Reduce to a steady simmer and cook 10–12 minutes. The flavors will meld, and the broth will deepen in color.
If it gets too thick, add the remaining ½ cup broth.
Add tofu and finish. Gently slide in the tofu cubes and simmer 3–5 minutes more to warm through. Drizzle in the sesame oil. Taste and adjust: add more soy for salt, a pinch more sugar for balance, or extra gochugaru for heat.
Garnish and serve. Turn off the heat and top with the green parts of the green onions.
Serve bubbling hot with steamed rice and, if you like, a fried egg on the side.