Slice the oranges in half and squeeze to make 500ml (17fl oz) juice. Pass the juice through a fine sieve into a pan. Cut both of the pomegranates in half and press one though a lemon squeezer to extract the juice, then add the liquid to the pan. Remove the seeds from the second pomegranate and set aside.
Soak the gelatine in cold water for 5-10 minutes until soft.
Add the sugar to the orange and pomegranate juice and heat over a medium heat until the sugar has dissolved and the juice is hot but not boiling. Remove from the heat. Drain the gelatine leaves, squeezing out any water. Add to the hot juice and stir until dissolved.
Pour the juice mixture into a bowl and leave to cool to room temperature. Add the chilled prosecco and gently stir. Once the jelly starts to thicken, add half the pomegranate seeds and ladle into glasses.
Cover and chill for at least 2 hours until set. Scatter the remaining pomegranate seeds over the top of each jelly to serve.
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