Sprinkle 1-2 Tablespoons of sugar over the top of the ½ of grapefruit.
Light the blowtorch, hold it close to the sugar on the fruit, and move it over the top of the fruit. It is important to keep the flame torch moving so that you do not light the skin of your grapefruit on fire.
When the sugar is bubbling and has turned a brownish color, turn off the torch and enjoy!