The fish purée that is the base of these delicate won-ton is easily bought in a Chinatown fish market, or you may make your own by grinding impeccably fresh fish in a food processor. Moist fish with a high oil content, such as sablefish (black cod) and salmon, work well. Use an additional egg white to bind the fish together if its own fats are not sufficient to form a gelatinous purée.
The won-ton may be filled and poached a day or two in advance of serving. Leftovers are yummy deep