Preheat the oven to broil on high.
Cut the grapefruit in half then use a small pairing knife to cut around each segment of grapefruit (so it is easier to get out).
Sprinkle each half with about 1 tsp of sugar, evenly across the top.
Transfer the grapefruits to a baking sheet and broil for 2-5 minutes, until the sugar has melted. Keep a close eye on the grapefruit so they don't burn!
Let them cool for a few minutes and enjoy while they're warm.