Heat oil in large skillet or wok. Add the ginger and garlic; saute for about 20 seconds . Add the black beans and the green onion and saute for about 10 seconds. Stir in the black bean paste and vegetable stock; mix well. Then add the tofu cubes. Turn the heat down to maintain a strong simmer; cover and let cook about 20 minutes. Add the cornstarch with water mixture until thickened and at last add sesame oil .