This sauce has a lovely tangy taste thanks to the tamarind, and works well with all types of seafood. We can recommend trying it with large king prawns.
Add onion and oil to a large saucepan or wok and cook over low heat for 5 minutes, until onion is soft. Add chillies and garlic and cook for another minute. Add dry spices for a further minute until fragrant, then add tamarind water and tomatoes. Simmer for 20 minutes then check for seasoning.
Turn heat up and add mussels to the pan or wok and place a lid on top. Stir once or twice and cook until all the mussels have opened, about 8 minutes. Discard any mussels which have not opened.
Serve immediately with roti or naan bread, if using.