Preheat broiler with rack 4 inches from heat. Using a vegetable peeler, remove 4 large strips of zest from one grapefruit (avoid bitter white pith). Cut into slivers; set aside.
Peel and cut both grapefruit into segments (see box). Place in a bowl. Squeeze membranes over a small saucepan; add any juice from bowl. To pan, add rosemary sprig, sugar, grapefruit zest,1/2cup water and 1/4teaspoon coarse salt. Boil until syrupy, 8 to 10 minutes. While syrup is cooking, rub fillets with oil. Season with salt and pepper. Place on a broiler-proof, rimmed baking sheet. Broil until opaque throughout, 7 to 10 minutes.
When syrup is done, discard rosemary sprig. Add syrup and chopped rosemary to bowl with grapefruit segments. Toss gently. Place on top of fish and serve.
Nutrition information per serving:
Calories251Fat6 gSodium238 mgSaturated fat1 g