Preheat the oven to 500 degrees F.

Toast the shrimp shells in a dry saucepan over medium-low heat until fragrant and color has developed. Cover the shells with water, add a pinch of salt, and simmer for 45 minutes over low heat, to form a stock. Strain the stock through a sieve and set aside.

In a large pot over low heat, sweat the onion, garlic, chile, ginger, and lemongrass in 2 to 3 tablespoons butter for 2 minutes. Add the red pepper and sweat for 1 minute more. Add the okra and shrimp stock. Simmer the mixture for 30 minutes, adjusting the seasoning as desired. Fold in the tomatoes, chopped cilantro, remaining 2 tablespoons butter, lime juice, and cashews. Remove from heat and reserve.

Heat a large oven-safe skillet over medium-high heat. Add enough oil to thinly coat the bottom of the skillet, and let oil heat. Add the fish, skin side down, and sear for 3 minutes. Then place the skillet in the oven and roast the fish for several minutes, or until it reaches your desired level of doneness.

Divide the reserved stew mixture among 4 bowls, and top each with 1 fish fillet, skin side up. Garnish with cilantro sprigs.