Rinse and scrub mussels under cold water. Discard any mussels that are crackd or that do not close when tapped gently. Set aside in fridge.
Heat oil in large pot over medium high heat. Add onion and cook until tender.
Add ginger, lemongrass, garlic and cook until it begins to brown about 3 minutes.
Add wine and scrape bottom of pan until all brown bits are incorporated. Turn up heat and boil until wine is reduced by half.
Add fish sauce, water and bring to boil. Reduce heat to low. Add mussels and then cover. Simmer until mussels have opened, about 3-5 minutes. Remove pot from heat. Stir in spinach and let heat of soup wilt the greens.
