Poach the pork. Put the whole piece of pork loin in a pot and cover with cold water. Add the ginger slices, the scallion, and 1 tbsp of the Shaoxing wine. Bring to a gentle simmer and cook 15 to 18 minutes, until just cooked through. Keep it at a lazy simmer, not a hard boil, or the lean meat dries out.
Lift the pork out and let it cool. For easier slicing, chill it in the freezer for about 15 minutes to firm up. Slice as thin as you can manage, around 1/8 inch, cutting across the grain.
Mix the sauce. Stir together the doubanjiang, sweet bean sauce, chopped black beans if using, sugar, soy sauce, and the remaining 1 tbsp Shaoxing in a small bowl.
Heat the oil in a wok or wide skillet over medium high. Lay the pork slices in a single layer and let them sear undisturbed for about 2 minutes, until the edges curl and turn light golden. Push the pork to one side of the pan.
Add the garlic and julienned ginger to the oil and cook 20 seconds. Add the sauce and fry it 30 seconds, until it smells deep and the oil turns red.
Add the leeks and bell pepper. Stir fry everything together 2 to 3 minutes, until the vegetables are crisp tender but still bright. Taste, and add a pinch of sugar if your bean paste is very salty. Serve hot over rice.