Cut the Japanese green peppers in half lengthwise. Remove the stems and seeds. Set aside.
In a bowl, combine the minced beef, unagi sauce, garlic, spring onion, and black pepper. Mix well by hand until the mixture becomes sticky and evenly combined.
Cover and chill in the fridge for at least 30 minutes.
Stuff each pepper half generously with the beef mixture.
Heat a pan over medium heat and add a little oil. Place the peppers meat-side down and sear for 4-5 minutes.
Add 3 tbsp water, then cover immediately with a lid. Let them steam for 2-3 minutes, allowing the peppers to soften and the filling to cook through — much like cooking gyoza.
Cook until the water has evaporated and the meat is nicely browned with lightly charred edges.
Remove from the pan and serve immediately while hot and juicy.