In addition to its rich supply of vitamin C, grapefruit provides potassium, dietary fibre, folate and, if it's ruby or pink grapefruit, some beta-carotene as well.
Peel the grapefruit with a small paring knife. Working over a bowl to catch the juice, separate the grapefruit segments from the membranes; reserve any juice that collects in the bowl.
Combine the redcurrant jelly, ginger, allspice and a pinch of pepper in a small frying pan. Bring to a simmer and cook for 1 minute.
Pour the mixture into a medium bowl and stir in the lemon juice and 1 tablespoon of the reserved grapefruit juice. Add the grapefruit and toss to combine. Serve chilled.