Heat the oil in a large heavy based saucepan. Add half of the coconut milk and cook over a medium heat until the milk starts to split.
Add the green curry paste and stir to combine, then add the lemongrass, ginger and chilli. Cook for about 10 minutes until the liquid has reduced by half.
Add the mussels, cover with a lid and cook over a medium heat for about 3 minutes.
Fold in the remaining coconut milk, lime juice, lime leaves, spinach and fish sauce and cook for a further 3 minutes.
Fold in the basil and discard any mussels that have not opened.