One of the secrets of steaming sole is to season the fish just before cooking. If the fish sits too long with the seasonings, the salt will extract water from the fish and, consequently, the fish will be less juicy. Try to choose a fish not longer than 14 inches, so that it will hang over the plate by no more than 1 1/2 inches on each end but will still fit in the steamer.
It is not necessary to purchase lemon sole for this recipe; just be sure the fish is very fresh and has the head and tail intact.