Preheat the oven to 200°C (approximately 390°F). Line a baking sheet with parchment paper.
Halve the grapefruits and with a grapefruit knife or a sharp paring knife cut around the segments and then in between to release the fruit from the membranes.
Pour the cumin seeds on the baking sheet and lightly toast. Transfer to a mortar and finely crush.
Place the halves cut side up, on the baking sheet and sprinkle with cumin seeds and sugar.
Bake for 10 minutes and serve immediately. Garnish with a sprinkle of sugar.