How to Make It
In a small saucepan, cover the tamarind pulp with 1 1/4 cups of water and bring to a simmer. Remove the sauce-pan from the heat and let stand, stirring occasionally, until the tamarind pulp is softened and broken down, about 15 minutes. Pass the tamarind through a fine sieve, pressing hard to extract as much pulp as possible. Discard the fibers and seeds in the sieve.
In a small skillet, toast the cumin and coriander seeds over moderate heat until fragrant, about 3 minutes. Transfer the spices to a plate to cool, then grind them finely in a spice grinder. In a blender, combine the ground spices with the coconut milk, peanuts, ginger, harissa and all but 1 tablespoon of the tamarind puree and blend to make a smooth, aromatic puree.
Melt 2 tablespoons of the butter in a large saucepan. Add the fish bones and cook over moderate heat until the fish on the bones is opaque, about 10 minutes. Add the scallions and garlic and cook until softened, about 3 minutes. Stir in 6 tablespoons of the aromatic puree (see Note). Add the remaining 2 cups of water and bring to a boil, then reduce the heat to low and simmer for 20 minutes. Strain the stock into a small saucepan and boil until reduced to 1 cup, about 10 minutes. Cover the stock and keep warm.
In a medium saucepan of boiling salted water, simmer the pearl onions over moderate heat until they are tender, about 6 minutes. Drain the pearl onions and return them to the saucepan. Add 1 teaspoon of butter, shake the pan to coat the onions and keep them warm.
Season the fish fillets with salt and pepper and set them, skin side up, on a heatproof plate that fits snugly in a large, deep skillet. Put 3 balls of crumpled foil in the skillet, add 1 inch of water and bring to a boil. Carefully set the plate on the foil balls, cover the skillet and steam the fish over high heat until the flesh flakes easily, about 8 minutes.
In a blender, combine the stock with the lime juice, the reserved 1 tablespoon of tamarind puree and the remaining 2 tablespoons of butter and blend until frothy. Season with salt and pepper and blend again. Set the fillets in soup plates and pour the tamarind sauce around them. Stir the mint into the pearl onions, arrange them around the fish and serve.
The remaining aromatic puree can be refrigerated for 1 week and used to marinate seafood or chicken.