First you filet the sole and make your sauce. Remove all the head and bone, vegetables trimming and add to a pot and fry with a touch of butter. When brown add in a full cup of chicken or fish stock and let it boil down to half a cup.
Then add in a full cup of champagne. Let it boil down to half and season with salt & pepper.
Add in a beaten egg white. The egg whites will traps any impurities. Sieve the stock and use that as your poaching liquid. Please refer to pics below.