1. Preheat broiler with rack 4 inches from the heat. Using a vegetable peeler, remove 4 large strips of zest from one grapefruit, avoiding the bitter white pith. Cut into slivers; set aside.

2. Slice the top and bottom of both grapefruits with a paring knife. Set one of the flat sides on your work surface. From top to bottom, following the curve of the fruit, cut away peel and white pith. Trim off any remaining pith. Holding the grapefruit over a bowl, cut along both sides of each segment, staying close to the membrane, to release. Squeeze membranes over a small saucepan and add any juices that have accumulated in the bowl. Add rosemary sprig, sugar, grapefruit zest, 1/2 cup water, and 1/4 teaspoon coarse salt. Place saucepan over medium-high heat and bring to a boil. Cook until syrupy, 8 to 10 minutes.

3. Meanwhile, rub fish fillets with oil; season with salt and pepper. Place on a broilerproof rimmed baking sheet; broil until opaque throughout, 7 to 10 minutes.

4. 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.