Heat the oil in a large shallow saucepan, add the spring onions, red pepper, garlic and ginger and fry for 2 minutes. Stir in the curry paste and cook for 1 minute, then mix in the coconut milk, stock, fish sauce and lime rind. Bring to the boil and simmer for 5 minutes.
Snip half the coriander into the soup using scissors. Add the mussels then cover and cook for 8–10 minutes until the mussels have opened.
Scoop the mussels out of the soup with a slotted spoon and put on to a large plate. Discard any shut mussels then reserve half the opened mussels in their shells for garnish. Take the remaining mussels out of their shells and stir them back into the soup. Ladle the soup into bowls, top with the reserved mussels in shells and garnish with the remaining snipped coriander. Serve with warm crusty bread for dunking into the soup.