Place the noodles in a heatproof bowl and cover with boiling water. Leave for 5 minutes, then drain.
Heat 2 tablespoons of the oil in a large saucepan. Add the shallots, chilli, lemon grass and ginger and fry over a gentle heat until the shallot has softened. Add the coconut milk and stock and bring to the boil, then reduce the heat and simmer for 5 minutes to infuse the flavours. Season the soup by adding the fish sauce, the juice of 1 lime and the sugar. Adjust quantities to taste.
Add the drained noodles and the monkfish to the soup and cook for 2 minutes. Discard any mussels that don't shut when tapped, then add to the soup. Cook until they open and the monkfish is firm.
Meanwhile, heat the remaining oil in a frying pan. Season the mullet with salt and pepper and fry, skin-side down, for 3 minutes, or until the skin becomes crispy. Turn the fish over and cook for a further minute. Squeeze the remaining lime juice over the fish.
Divide the soup between 4 bowls, removing any mussels that have not opened, and top with the mullet.