Combine the cumin seeds, crushed ginger, crushed garlic, cardamom pods, turmeric, dried red chilies, Maldivian chili, thinly sliced 3/4 onion, scraped fresh coconut, and 1 tbsp salt in a bowl.
Heat a dry frying pan over medium heat and roast the spice-coconut mixture briefly, stirring, until it browns lightly and becomes fragrant; remove from heat.
Drain any excess liquid from the roasted mixture, transfer to a food processor, add about 1/4 cup (60 ml) water, and grind to a smooth paste.
Prepare the tuna by skinning and deboning, then slice into pieces about 1 cm thick and roughly 10 cm square.
Place half of the prepared curry paste in a bowl. Spread about 1/4 tsp of the paste on one tuna slice, roll the slice up, then wrap each roll with a thin coconut leaf strip and tie or tuck to secure; repeat for all pieces.
In a pot, mix the light coconut milk with two spoonfuls of the curry paste and heat for a few minutes over medium heat.
Add the remaining 1 1/2 onions (sliced) and the 1 1/2 tsp salt, bring the mixture gently to a boil, then add the prepared tuna rolls. Reduce heat and simmer until the fish is tender, about 15–30 minutes depending on thickness.
Stir in the thick coconut milk and simmer until the curry thickens to your desired consistency, then remove from heat.
Serve the curry hot with rice.