Mix gelatine with reserved cooking liquid.
Bring to a boil, and boil, uncovered, until reduced to 3 1/2 cups.
Strain through cheesecloth into a bowl, and taste for seasoning.
Chill in refrigerator until syrupy, stirring occasionally, or set the bowl in a dish of ice cubes and stir until the right consistency.
This amount is sufficient to glaze an 8-pound fish.