Poaching fish in tart fruit juice — such as fresh cranberry — transforms the liquid into a dual-purpose cooking medium and finishing sauce. The acidity gently firms the fish's proteins while the natural sugars concentrate during cooking to create a vivid, deeply flavoured result. The key is maintaining the barest possible simmer: a rolling boil toughens delicate fish and clouds the juice. A small amount of added sugar tames bitterness before cooking begins. Once the fish is removed, reducing the liquid by half and mounting with cold butter creates a glossy restaurant-quality sauce from what began as raw juice.