1. Clean the mussels by rinsing in cold water and detaching any beards. Discard any open mussels that do not close when lightly tapped. Drain and put to one side in a large bowl.
2. Heat the oil in a heavy based pan and saute the garlic and shallots until soft, add the ginger, coriander, lemon zest and lemon grass, sweat out for 2-3 minutes.
3. Add the fish stock and fish sauce to the pan, then reduce gently. Season to taste.
4. Add the drained mussels to the pan. Cover and cook for 4-6 minutes or until the mussels open. Gently stir in the coconut milk and serve immediately sprinkled with the coriander leaves.