Heat the sesame oil in a wok or large skillet over medium-high heat, and cook and stir the ginger and garlic until the garlic just begins to brown, about 30 seconds. Stir in the pork, breaking it up as you stir, and cook and stir until the pork is browned and broken into small pieces. Add the chili bean sauce, soy sauce, sake, and oyster sauce, stir to mix the ingredients together, reduce heat, and let simmer until the mixture thickens slightly, about 3 more minutes.
Thin the sauce with water as needed (sauce should not be watery) and gently stir in the tofu. Simmer until the tofu is hot and coated with sauce, about 10 minutes, stirring a few more times. Serve sprinkled with chopped green onion.