Put the pork belly, water, ginger, and scallions in a saucepan. Bring to a gentle simmer and cook for 25 minutes, until a skewer enters the meat with slight resistance.
Cool, pat dry, then refrigerate for 1 hour. Slice across the grain into irregular 1/8-inch pieces.
Prepare the Sauce:
Mix the doubanjiang, black beans, soy sauce, sugar, and sesame oil in a small bowl.
Heat the wok until hot, add the neutral oil, then spread in the pork slices. Stir-fry for 3 to 5 minutes until the edges curl and some fat renders.
Second Cook:
Push the pork aside, cook the sauce mixture in the center for 30 seconds, then toss everything together.
Add the green garlic and peppers. Stir-fry for 2 to 3 minutes until just tender and serve immediately.
Notes
Pixian doubanjiang gives the dish its characteristic chile and fermented-bean flavor.
Use pork belly with clear alternating lean and fat layers.