2 tablespoons stem ginger syrup (taken from the jar)
Remove the skin from the grapefruits then, holding over a bowl to collect the juice, use a sharp knife to cut into segments between the pith. Place the segments in a serving bowl and chill.
To make the ginger syrup, pour the grapefruit juice into a jug to make 200 ml (7 fl oz), then place in a saucepan with the sugar, stem ginger and stem ginger syrup. Bring to the boil, then reduce the heat and simmer for 8–10 minutes. Transfer to a heatproof jug and chill for 10 minutes to cool.
Pour the cooled syrup over the grapefruit and stir in the mint. Serve immediately.