1. Pre-heat oven 375ºF.
2. On a sheet tray or in a skillet, roast the cardamom pods, clove, cumin seeds and coriander seeds until fragrant.
3. Transfer to a spice grinder; blend. Sift, add salt and pepper to taste, and set aside spice mixture.
For the sauce:
4. In a pan, heat oil. Add mustard seeds and cook until they begin to crackle.
5. Add curry leaf, onion, garlic, and ginger. Cook until soft, stirring to prevent coloration.
6. Add turmeric, and cook for another 30 seconds, stirring continuously.
7. Stir in yogurt to combine. Add salt and pepper to taste.
8. Remove from heat and allow to cool to room temperature.
9. Place halibut fillets on a sheet tray; sprinkle tops with spice mixture.
10. Cook in oven for 7-10 minutes, or untill cooked through; remove from oven. Place fillets on center of plate, ladle yogurt sauce around and serve immediately.