Preheat the slow cooker if necessary; see the manufacturer's instructions. Put the butter and sugar in a saucepan, add the fruit rinds, then strain in the juice. Heat gently for 2–3 minutes, stirring occasionally, until the butter has melted and the sugar has dissolved.
Pour the mixture into a basin that will fit comfortably in your slow cooker pot. Leave to cool for 10 minutes, then gradually strain in the eggs and mix well. Cover the basin with foil, put foil straps or a string pudding bowl lifter (see
Warm 2 clean jars in the bottom of a low oven. Spoon in the citrus curd, place a waxed disc on top and leave to cool. Seal each jar with a screw-top lid or a cellophane jam pot cover and an elastic band, label and store in the refrigerator. Use within 3–4 weeks.