Rinse the orange and lime very carefully under plenty of cold running water and pat dry on kitchen paper.
Remove the zest (peel) with a small sharp knife or potato peeler, taking care not to bring any of the pith with it.
Cut each of the fruit in half and squeeze the juice into two separate bowls.
Put the zest (peel) from both fruit into a cup of hot water.
Microwave on HIGH for 1 minute.
Drain the zest (peel) and plunge into cold water.
Put the cornstarch (cornflour) in a bowl and gradually stir in 1/4 cup (3 tablespoons) of lime juice and 1/4 cup (4 tablespoons) of orange juice until a smooth mixture is obtained.
Stir in the sour cream (creme fraiche) and salt and pepper to taste.
Microwave for 1 minute on HIGH.
Stir.
Microwave 1 further minute on HIGH.
Stir well and cook for 1 final minute.
Stir again.
Rinse the halibut under plenty of cold running water and pat dry on kitchen paper.
Cut in half lengthwise, then cut each half-fillet diagonally into strips about 3 inches (7.5 cm) long.
Sprinkle salt and pepper on to 4 dinner plates.
Arrange the strips of fish equally between them in a star shape.
Microwave each plateful on HIGH for 1 1/2 minutes.
Coat the fish with the citrus sauce.
Sprinkle with the zest (peel) and serve at once.