Preheat your broiler to high. Line a sheet pan with aluminum foil. Cut a thin slice of rind (don't cut into fruit) off each end of grapefruit so that halves will lie flat. Cut grapefruits in half and section each half.
Place halves, cut sides up, on sheet pan. Sprinkle half of the mint over fruit. Cover with maple sugar. Broil, 3-4 inches from flame, until sugar has melted and begins to bubble. Watch it closely, as sugar burns easily. Remove from broiler, top with remaining mint, and serve.