How to Make It
In a small skillet, toast the cumin over moderate heat until fragrant, about 30 seconds. Transfer to a plate to cool.
Heat the olive oil in a small skillet. Add the shallot and cook over moderate heat until softened, about 2 minutes. Add the clam juice and simmer for 3 minutes; transfer to a blender. Add an avocado half, the lemon juice and cilantro leaves and blend until smooth. Scrape the sauce into a small saucepan and season with salt and pepper.
Season the halibut with salt and pepper and sprinkle the cumin seeds on both sides, pressing to adhere. Add 3 inches of water to a steamer and bring to a boil over high heat. Transfer the fish to the steamer basket, cover and cook over moderate heat for 2 minutes. Uncover and spoon some of the tomatoes on top of each fillet. Cover and steam until the fish is just opaque throughout, about 3 minutes more.
Gently reheat the avocado sauce, but do not boil. Transfer the fish to plates and pour the sauce alongside. Garnish with the remaining avocado slices and cilantro sprigs and serve at once.