Preheat oven (electric cooker: 150°C/circulating air: 125°C/gas: see manufacturer). Whisk the egg yolks and 160 g sugar until light creamy. Stir starch with 300 ml juice until smooth. Gradually stir into the egg yolk-sugar cream.
Divide the mixture into four ovenproof moulds (each containing approx. 200 ml). Place the ramekins in an ovenproof dish. As much
Allow to stand in a hot oven for approx. 1 1⁄2 hours. Remove from the oven and let it cool down. Chill for at least 5 hours, preferably overnight.
To serve, sprinkle 1 tablespoon of sugar on each of the grapefruit flans. Caramelise with a kitchen gas burner, swinging the flame slightly back and forth at a slight distance so that the sugar does not burn.