How to Make It
In a medium saucepan of boiling water, cook the peas for 1 minute. Drain and rinse under cold water. Wipe out the saucepan.
In the same saucepan, melt 2 tablespoons of the butter. Add the shallots, lemongrass and garlic and cook over moderate heat, stirring occasionally, until softened but not browned, about 5 minutes. Add the wine and cook over moderately high heat until almost evaporated, about 5 minutes. Add the clam broth and water and cook until reduced to 1/2 cup, 10 to 12 minutes. Add the cream and cook until slightly thickened, 2 to 3 minutes. Transfer the sauce to a blender and puree until smooth. Strain the sauce into a small saucepan, pressing hard on the solids, and season with salt and pepper.
In a large nonstick skillet, heat the olive oil. Season the fish with salt. Add the fish to the skillet skin side down and cook over moderately high heat until golden, about 3 minutes. Turn the fillets and cook until the fish is just white throughout, 2 to 3 minutes longer.
In a small saucepan, melt the remaining 2 tablespoons of butter. Add the peas and warm over low heat; season with salt and pepper. Gently reheat the sauce. Spoon the sauce into plates or shallow bowls. Top with the peas and the fish. Sprinkle with the mint and serve.