Place the lemon grass, shallots, chilli, garlic, ginger and oil in a food processor and blend to a smooth paste.
Heat a saucepan over a medium heat and fry the paste for 2 minutes until fragrant, then add the coconut milk. Bring to the boil and cook for 5 minutes until it reaches the consistency of cream. Add the sweet potatoes and cook until tender.
Add the monkfish when the potato has almost cooked, and simmer for a further 5 minutes, or until the fish becomes firm. Finally, add the fish sauce, sugar and lime juice. Taste and adjust the amounts of these flavourings to taste, then garnish with the coriander and serve with some Thai sticky rice.