Heat oil in a pot set over medium high-heat. Add garlic, ginger and chilies and cook until fragrant, about 30 seconds. Add black bean sauce and water and bring to a simmer. Add mussels, cover, and cook until they just open. Transfer mussels and their cooking liquid to serving bowls. Top with green onions.
Note: Buy fresh mussels the day you intend to use them. To prepare them for cooking, first rinse in cold water. Drain well and discard any that do not close when tapped. Discard any that do not open after cooking. Both are indications the mussel did not survive the trip home from the supermarket and is not fit to eat.