In a small bowl, mash the black beans into the garlic and ginger. In a large wok or saute pan heat the oil and cook the pork, stirring occasionally until all the pink is gone--about 10 minutes. Then add the mashed fermented beans and stock, mix together and cook for 2 minutes. Add the green beans, mix together and cook for 3 more minutes. Meanwhile, in a bowl mix together the soy sauce, 2 tablespoons of water and cornstarch until they form a paste. While stirring pour the cornstarch mixture into the liquid in the bottom of the wok. The liquid will thicken and as you toss the wok, coating the beans. Taste and re-season with salt and pepper if needed