1 grapefruit (preferably Ruby Red)
2 tablespoons sesame seeds
1 teaspoon granulated sugar
1½ teaspoons kosher salt, divided, plus extra for serving
Four 6- to 8-ounce halibut fillets
12 radishes, trimmed and quartered lengthwise
8 scallions, sliced on a bias into 1½-inch-long pieces
2 tablespoons plus 2 teaspoons canola or grapeseed oil, divided
¼ teaspoon freshly ground black pepper
1 lime, halved
1. Preheat the oven 475°. Set the grapefruit on a cutting board. Use a Microplane-style grater to remove 1 tablespoon of the grapefruit zest, placing it in a small bowl. On a cutting board, slice off the ends of the grapefruit and stand it upright. Slice off the peel in long strips, working from top to bottom and removing all of the white pith. Hold the grapefruit over a medium bowl and slice between each membrane to release the grapefruit segments into the bowl. Drain off the juice (drink or reserve for another use), then slice each grapefruit segment in half on a bias.
2. To a spice or coffee grinder, add the sesame seeds and pulse until medium-fine. Add the sugar and ½ teaspoon of the salt and pulse to combine, then add the sesame mixture to the bowl with the grapefruit zest, using your fingers to rub the mixture together. Place the halibut fillets on a large plate, season with ½ teaspoon of the salt, and evenly sprinkle the sesame-grapefruit rub over the surface of each fillet, pressing down so the rub adheres.
3. Line a rimmed baking sheet with aluminum foil. In a large bowl, add the radishes, scallions, 2 teaspoons of the oil, the remaining ½ teaspoon of salt and the black pepper and toss to combine. Turn the radish mixture out onto the foil-lined baking sheet, moving it to one side of the sheet pan but keeping it in an even layer (the halibut is roasted on the other side). Roast the vegetable mixture until the edges of the vegetables just start to brown and color, about 6 minutes.
4. While the vegetables roast, start cooking the fish: In a medium skillet set over medium-high heat, add the remaining 2 tablespoons of oil. Once the oil ripples, after about 1 minute, add the halibut fillets, seasoned side down. Cook just until the sesame-grapefruit rub is crusted and golden-brown, 30 seconds to 1 minute. Immediately remove the pan from the heat. Carefully slide a metal spatula under each fillet and transfer them to the baking sheet in the oven, setting the fillets rub side up next to the vegetables. Continue to roast until the sides of the fish are mostly opaque and when pressed, the halibut resists light pressure but isn't completely firm, 4 to 6 minutes longer.
5. Remove the baking sheet from the oven and use a spatula to return the vegetables to the large bowl. Add the grapefruit segments and the juice from ½ lime, gently tossing to incorporate. Taste and season with salt if needed. Divide the grapefruit-radish salad among 4 plates and place 1 halibut fillet on each plate. Squeeze the remaining ½ lime over the fish, sprinkle with salt, and serve.