Use a sharp knife to peel the oranges and grapefruit. Carefully slice away all the pith and cut into manageable slices – they shouldn’t be too thin.
Place the sugar in a small saucepan with enough water to just cover. Bring to a boil, then simmer until it forms a light blond syrup. Remove from the heat and add the orange and lemon juices. Be careful that it doesn’t spatter and burn you.
The sugar will harden, but will dissolve again as you stir. When the syrup has cooled a little, add the chilli and ginger. Once it has cooled completely, add the lemongrass and leave the syrup to infuse.
Pour the cold syrup over the sliced fruit and leave to marinate for a few hours in the fridge. Serve chilled or at room temperature, as preferred.
Serve chilled or at room temperature, as preferred.