Combine sugar and juice and gently simmer until sugar is completely dissolved.
Add the lemon zest and rum. Melt the gelatine (follow the instructions on the package) in a little hot water.
Add the juice to the gelatin form, stir well, pour into cooled rinsed ramekins or a glass bowl form. Place in the refrigerator to set. Turn out onto a cold plate, garnish with whipped cream.
(Rueegg/Feisst: “Grossmutters Mittelmeerküche – Kochen wie im Urlaub”, Albert-Müller-Verlags-Ag)