In a bowl mix together the ginger, oyster sauce, hoisin sauce, dark soy sauce, sugar, and water. Heat wok over medium heat. Add oil. When the oil is hot enough, add the onion and stir-fry until soft. Dont brown it. Add the pork and stir-fry for 30 seconds, pour in the sauce mixture and bring to a boil. Stir the cornstarch/water adding it slowly to the pork, then cooking and stirring until thick. Add the sesame oil, cook another 10 seconds and remove from heat. Let cool completly before stuffing into buns. The mixture should be more on the thick rather than runny side.