How to Make It
In a medium skillet, melt 1 tablespoon of the butter. Add the shallot and cook over low heat until softened, 5 minutes. Add the clam juice and water and boil until reduced to 1/2 cup, 4 minutes. Add the sauerkraut and cook just until heated through. Remove the skillet from the heat and swirl in the remaining 5 tablespoons of butter, 1 tablespoon at a time, putting the skillet back over moderate heat briefly once or twice, as necessary. Stir in the pickle and season with salt and pepper.
In a large skillet, heat the oil. Season the mahimahi with salt and pepper and cook over moderately high heat until nicely browned on the bottom, 3 minutes. Turn the fish and cook until just white throughout, 2 minutes longer.
Very gently rewarm the sauce, whisking constantly over low heat. Stir in parsley and spoon the sauce onto plates. Set the fillets on the sauce and serve.