In a large saucepot or wok, over medium-low heat, begin to toast the star anise, cinnamon, cardamom, and mustard seeds for approximately 4 to 5 minutes.
Remove the aromatics and place in cheesecloth. With butchers twine create a sachet or pouch and tie it to the handle of the pot. The aromatic sachet should now be in the pot and tied firmly to the handle.
Add the peanut oil, ginger, garlic, chilies, lemongrass, and leeks to the saucepot with the aromatic sachet, and over medium heat cook until translucent, about 2 to 3 minutes.
Add the mussels, mangosteens, and curry paste and with a wooden spoon begin to work these ingredients into the spices for 1 minute.
Add the coconut milk, tamarind water and kaffir lime leaves.
Bring to a low boil and reduce to one-third of its original mass, approximately 30 minutes.
While the curry is reducing take the fish and remove the heads if desired. For better flavour, keep the heads intact, but remove the eyes with a small spoon.
With a sharp knife, score the fish three times, about a 0.50 inch deep, down the entire length of the fish, then repeat on the other side.
Once the curry has reduced, add the fish and cook for 10 minutes.
Serve over steamed rice and sprinkle with fresh limejuice, Thai basil, and cilantro.
Season with sea salt and black pepper to your liking.