Cut the grapefruit in half then cut around each piece of grapefruit. Sprinkle the brown sugar evenly between the two halves.
Using a creme brûlée torch, melt the sugar on the grapefruit. If you don't have a creme brûlée torch, set the oven to broil on high (or 450 degrees) and broil for 5-7 minutes, until the sugar is melted.