Turn broiler on high (525°F). Line a baking sheet with foil, if desired, and place grapefruits on the pan. To help them stand flat, you may need to cut off ~1/4 inch from the rounded bottom.
Brush tops of grapefruits with maple syrup (alternatively you can use honey or brown sugar). Broil for 4-5 minutes until slightly bubbly and caramelized - watch carefully so flesh doesn't burn (rind may brown).
Meanwhile, in a small bowl combine sugar and mint, rubbing between fingers to release the oil from the mint.
Top grapefruit with your desired amount of sugar mixture and serve immediately.
Notes
I used raw sugar in the photos but you can also use regular granulated sugar! The sugar amount is approximate, depending on how much you'd like on your grapefruit.
To make eating easier, before broiling run a knife along the edge of the flesh and rind and then between the membrane to segment the pieces. Don't go all the way through the rind, though! You just want to loosen it up enough to be able to spoon it out.