Heat the pear cider, caster sugar and lemon zest in a heavy-based saucepan over medium heat to simmering point, stirring to dissolve the sugar. Add the pear halves and cover closely with a cartouche (sheet of baking paper) and small plate to keep the pear submerged in the liquid, then cover the cartouche with a lid. Reduce the heat to low and simmer gently for 30–40 minutes