Preheat oven to 375°F. Coarsely grind 1 tablespoon coriander seeds in a spice mill or with a mortar and pestle. (Alternatively, you can coarsely chop with a knife.)
Toss leeks, cilantro sprigs, oil, half of lemon slices, and 2 teaspoons ground coriander in a large roasting pan; season with salt. Roast, tossing occasionally, until leeks are tender and starting to brown, 15-20 minutes. Remove roasting pan from oven and carefully pour infused oil into a large heatproof measuring cup.
Reduce oven temperature to 275°F. Season halibut with salt and arrange over leeks in roasting pan. Top with remaining lemon slices and ground coriander and pour infused oil over fish. Roast until halibut is just cooked through and starting to flake, 30-35 minutes.
Cut halibut into large pieces and serve with leeks and lemon topped with chopped coriander seeds and cilantro leaves.
Do ahead: Halibut can be roasted 1 hour ahead. Let cool and cover.
