(photo via the Running Foodie)
I have come across several similar recipes, such the one at "This American Wife" which includes butter and cinnamon in addition to the brown sugar. I prefer to only add a small amount of brown sugar and my roommate prefers to have hers plain.
Recipe (if you can even call it that!)
1. Cut grapefruit in half and set into an oven-safe dish.
2. Sprinkle on your chosen amount of brown sugar.
3. Place in oven and broil until sugar has melted and is slightly bubbling.