Place the coconut milk into a large pan with the lemongrass, ginger and garlic. Chop the coriander stems using a sharp knife and add to the pan. Bring the mixture to the boil, turn down the heat and simmer for 4 minutes. Add the chopped chilli, coconut cream, mussels and fish sauce to the pan and cover with a tight fitting lid or baking tray so that the mussels can steam. Bring the liquid back to the boil, then simmer for a few minutes, shaking the pot every so often so the mussels move around and open up. When all the mussels are open, remove the pan from the heat and stir in the lime juice. Season to taste with salt and pepper before quickly serving the mussels in 2 deep serving bowls. Spoon the delicious juices over the top and sprinkle over the coriander leaves before serving.