This is a refreshing citrus treat for dessert, especially when served alongside our Whole Wheat Walnut-Raisin Biscotti.
Preheat broiler with rack 4 inches from heat. Remove peel and pith from grapefruit. Halve fruit vertically; slice crosswise into 3/4-inch-thick half-moons. Place pieces, overlapping to fit, in a shallow broilerproof baking dish (ours was about 12 by 7 inches).
In a small bowl, stir together sugar and cinnamon; sprinkle over grapefruit. Broil until sugar has begun to brown, 15 to 20 minutes. Serve with biscotti, if desired.