Put the testicles in a saucepan of cold water and bring to the boil. Blanch the testicles for 2 minutes, then cool under running water. Cut into thick slices.
Heat the duck fat in a frying pan, put in the testicles and fry over not too high a heat for 3 minutes on one side. Turn them over and cook for another 3 minutes. Lift them out with a slotted spoon, transfer to a dish and keep war