Crack the cardamom pods open using a pestle and mortar or other heavy weight.
Put the orange and grapefruit juice into a stainless steel saucepan and add the lemon juice and the strips of orange peel.
Add the cardamom pods and sugar and stir until the sugar has dissolved.
Bring the liquid almost to the boil, then turn off the heat and leave to cool a little, covered with a lid. This will let the aromatics to perfume the juice.
Meanwhile, soften the gelatine for a minute or two in cold water. It will turn to a soft, quivering blob.
When the juice has cooled a little but is still warm, drop in the gelatine and stir.
When it has dissolved, pour the jelly through a sieve into glasses and refrigerate.
You can leave a few cardamom pods in each glass if you wish.
To make the sugared zest, put each piece of orange rind flat on a chopping board and slice into very fine hair-like strips with a small knife.
Put the rind in a non-stick pan with the sugar and a couple of tablespoons of water. Leave over a low to moderate heat until the sugar has melted. Keeping a close eye on its progress, wait till the water has evaporated and the sugar has just started to turn golden.
Tip the sugared peel out onto a piece of greaseproof paper and leave to cool and set. To serve, break the sugared peel into pieces and place a few on top of each glass of jelly.