Defrost mussels. Shell and wash. Drain well. cut fillet into thick slices. Add marinade ingredients. marinate for 10 minutes. Scald in boiling water and drain well.
Crush lemongrass, onion, garlic, shallot and chili. Put into a mixer. Add 1 cup of water. Blend well. strain. Set water aside.
Heat 2 teaspoons of oil. Sauté tomato paste over low heat until fragrant. Add strained water, seasoning, stock and coconut juice. Bring to the boil. Try the taste. Put in mussels and fillet. Boil again. Dish up. Serve.