Add the shallots, ginger, garlic, lemongrass, and chili, and stir-fry until golden and aromatic, about 3 minutes. Add the water, fish sauce, and sugar, stir and bring to a simmer. Divide the mussels and broth evenly among the bowls.
In a wide saucepan, heat a little groundnut oil and soften the spring onion, garlic, coriander stalks, lemongrass and most of the red chilli for around 5 minutes. Add the coconut milk and fish sauce and bring to the boil, then add the mussels and cover the pan.
Large green-shelled mussels are farmed along the coast of New Zealand and Stir in the brown sugar, Asian fish sauce, unsalted butter and minced cilantro.