Preheat the grill to high.
Mix chilli powder, ground coriander, turmeric, tamarind, and salt.
Rub the spice mix into the flesh side of the fish fillets and marinate.
Fry cumin, cardamom, bay leaf, cinnamon, cloves, and peppercorns in oil for 3 minutes.
Add chopped onions and fry for 5 minutes until lightly browned.
Stir in ginger-garlic paste and fry for 3 minutes.
Add tomatoes, turmeric, coriander, and chilli powder then cook for 10 minutes until thick.
Stir in crabmeat, season with salt, and add chopped coriander.
Grill fish fillets skin-side down for 2 minutes until firm.
Top half of the fillets with crab and cover with remaining fillets to create sandwiches.
Grill sandwiches for 5 minutes, turning once, until the skin is crisp.
Drizzle with a whisked mixture of vinegar, olive oil, and turmeric before serving.