Directions: In a heavy skillet, large enough to hold all the fish in one layer, (optional, but helps keeping fish pieces together during cooking and when removing), bring two cups of water, the onion, sugar, salt and pepper to the boil, adding the sugar at this point greatly improves the fish, without noticeably sweetening it. Lower the heat while you prepare the fish.
Add the cleaned fish to a skillet. Bring the poaching liquid back to the boil, then lower the heat so it is barely bubbbling. Partly cover and simmer very gently for 20 minutes.
Lift out the fish with a slotted spoon, draining any liquid back into the skillet. Remove the skin, but leave the bone. Place the fish in an oval serving dish, deep enough to hold the fish and liquids comfortably, then leave to cool while you make a sauce.
To make the sauce, boil the remaining fish-poaching liquid for 5 minutes to concentrate the flavor, then strain it and measure out one cupful.
Beat the eggs thoroughly, then whisk in the cup of cooled fish liquid, then lemon juice and the potato starch cream. Alternatively, mix all ingredients for 10 seconds in blender or a food processor (this makes it easier to thicken the sauce , without fear of its curdling, I use the latter method with great success). Put this liquid into the heavy-based saucepan and cook gently over slow heat until the sauce thickens to a consistency of a coating custard, you will need to steer it constantly. Do not let it boil, or the eggs may curdle!
Taste the sauce, and add extra lemon juice if necessary to make it equally sweet and sour. Pour the sauce over the fish, coating it completely. Wrap tightly in a plastic wrap and then in aluminium foil, (precaution for reventing the fishy smell escaping) and refrigerate overnight or up to four hours.
Serve at room temperature, garnished with parsley and lemon wedges.