Fresh mussels with chilli and coconut

1kg fresh tightly closed mussels

2 tbsp sunflower oil

1 onion, peeled and finely chopped

2 garlic cloves, peeled and crushed

1 small red chilli, seeded and finely chopped

1 tbsp Thai green curry paste

150ml fish stock

150ml coconut milk

sea salt 

black pepper

squeeze of lemon juice

1 tbsp coriander leaves, roughly torn