Squeeze 2-3 oranges.
Peel one orange with a sharp knife, removing all the white pith and membrane.
Then slice the orange.
Strain the blood orange juice through a sieve, then put into a pan with the sugar and heat.
Soak the gelatine in plenty of cold water.
Then squeeze out and dissolve in the hot juice.
Stir in the Prosecco.
Pour into 4 glasses and leave to cool.
As soon as the liquid begins to set, place a slice of orange on top of each and put into the refrigerator for about 3 hours to set.
Whip the cream with 1 tsp sugar until stiff.
Put a small spoonful on top of each portion of jelly and serve garnished with lemon balm.