Roast all the spices for 3-4 minutes. Grind them to a fine powder, add in the vinegar, 2 cloves of garlic and blitz it up to a fine paste. Marinate the prawns and fish with 1/2 tsp of this paste and keep it aside for 15-20 minutes.
Heat oil in a heavy pot add in the onion, kaffir lime leaves and lemongrass stick. Saute the onion till it softens.
Add in the rest of the garlic, red chilli powder, spice paste, tomatoes, coriander stalks and cook till the onion is nice and brown, tomatoes are cooked and release oil.
Now tip the mussels in, saute till they open. Fish them out with a slotted spoon and keep it aside.
Add in the boiled potatoes and toss them well with the masala.
Add in the salt, coconut milk, tamarind water and water, bring it to a gentle boil. Adjust the seasoning.
Add in the prawns, fish and the mussels and cook for 4-5 minutes till they are cooked.
Garnish with fresh coriander and chilli oil if you prefer. Serve hot with plain rice.