How to Make It
In a medium bowl, toss the fish with 1 tablespoon of the lime juice. Cover and refrigerate for 1 hour. Drain and pat dry.
In a large pot, heat the oil until shimmering. Add the fish in a single layer and brown on one side only over moderately high heat, about 1 1/2 minutes. Transfer to a plate.
Add the onion, chiles, garlic and the 1/4 cup chopped cilantro to the pot and cook, stirring, until the onion is translucent, about 1 minute. Stir in the tomato paste, coating the vegetables. Add the clam juice and scrape up any browned bits on the bottom of the pot. Bring to a boil, then reduce the heat to moderate and simmer for 4 minutes. Pour in the coconut milk and simmer until thickened slightly, about 3 minutes.
Return the fish to the pot. Add the shrimp, cover and cook over moderate heat, stirring occasionally, until shrimp are almost cooked through, about 2 minutes. Nestle the mussels into the stew, cover, and cook until the mussels have opened, about 2 minutes.
Whisk the cashew butter with 2 tablespoons of hot water, then stir it into the stew. Add the remaining 2 tablespoons of lime juice and season with salt. Transfer the stew to bowls, garnish with cilantro and serve.