Scale and clean the fish thoroughly, remove the gills, which are always muddy, sprinkle the fish liberally with lemon juice, reserving 1 tablesp. of juice, put the fish aside for 1 hr. Then melt the butter in a baking-dish, put in the fish and baste it well, sprinkle with salt and pepper and add the finely-chopped shallots. Cover with greased paper and bake gently for 25–35 min., according to taste. Add the gherkin and 1 tablesp. lemon juice to the sauce and season to taste. Serve the fish with the sauce poured over.