Blitz half the red chilli, the coriander stalks, garlic, ginger and coconut milk in a blender.
Pour the coconut milk mixture into a saucepan, add the lemongrass and cook over a medium heat for 5 mins. Turn up the heat, add the prawns and mussels, then cover and cook for 4 mins – the prawns should be pink and the mussel shells should be open (discard any that have not opened). Remove the lemongrass and discard.
Divide the shellfish and broth between two serving bowls and squeeze over some lime. Chop the coriander leaves and sprinkle over the top along with the rest of the chilli. Serve with slices of crusty bread to soak up the broth.