Step 1In a small, dry saute pan over low heat, toast the coriander and cumin seeds until fragrant, about 30 seconds. Grind in a spice grinder.
Step 2In small bowl, combine the ground coriander and cumin seeds, yogurt, cucumber, garlic, shallot, mint, cilantro, cayenne pepper, lime zest and juice and salt and pepper to taste. Chill. (Makes 1 1/2 cups.)
Step 1In a saucepan, bring the stock, mint and lime zest to 180 degrees (small bubbles will be forming around the edges of the liquid and steam will be rising). Place the halibut in a ceramic or glass baking dish that's at least 2 inches deep and pour the hot liquid over the halibut. The halibut should be covered by the stock.
Step 2Let the fish stand (turning the pieces after 5 minutes) in the baking dish for 1 hour, 5 minutes, until the stock and fish have cooled to room temperature (about 72 degrees). (Do not refrigerate or the fat will congeal and the mouth feel will be waxy.)
Step 3When the fish has reached room temperature, remove it from the liquid and transfer it to a serving plate. Season with salt and pepper and serve with the yogurt sauce.