Scrub mussels under cold running water and remove beards. Place with 1 cup water in a large saucepan, cover and bring to the boil. Reduce heat and simmer 2 minutes. Drain mussels and remove from shells. Discard any that do not open.
Heat butter in a frypan, add chilli flakes, ginger, garlic, lemongrass and green onions. Fry about 2 minutes to soften the onions.
Add coconut milk, fish sauce and sugar. Bring to a boil and simmer 1 minute. Add mushrooms and cook a further minute. Blend the flour and extra tablespoon water together and stir in. Simmer until sauce thickens.
Stir in mushrooms and basil and heat through. Serve at once.