Make the sauce: dilute the mentsuyu with water to produce 50 ml liquid. Add the gochujang a little at a time, mixing after each addition. Then add the mirin and mix well. Don't worry if a few lumps remain.
Coat a frying-pan with vegetable oil as needed (not listed in ingredients above), and pot roast the bork-belly by first browning the meat, then adding the Step 1 sauce and simmering over medium heat.
Continue to cook until almost all the liquid has evaporated and the meat has absorbed the flavors, taking care not to burn it. When the meat becomes glossy, it's done.
Garnish with thin slices of Japanese leek, don't skip this finishing touch.
Story Behind this Recipe
I wanted to make something as quickly as possible, so just threw together whatever I had in the fridge. It turned out really delicious, so I decided to write down the recipe. I added gochujang because I just happened to have it in the fridge, but I don't think I would ever come up with this idea if it hadn't been there.
You'll end up going back for a second serving, and a third, and then a fourth, and so on. Be sure to use pork belly! Leaner cuts won't work as well.