Preparation

• Position a rack in the center of the oven and heat the oven to 500°F.
• In a small bowl, whisk the sour cream, yogurt, dill, lemon juice, and sugar. Season to taste with salt and pepper.
• Combine the breadcrumbs, scallion whites, oil, and a large pinch each salt and pepper in a medium nonstick skillet over medium heat. Cook, stirring occasionally, until the breadcrumbs crisp and the scallions brown, 10 to 12 minutes. Set aside to cool.
• Meanwhile, lightly oil a large rimmed baking sheet. Place the fish on the baking sheet, season with salt and pepper, and drizzle with olive oil. Roast until firm and just opaque throughout, 5 to 8 minutes, depending on thickness. Serve the fish topped with some of the sauce, breadcrumbs, and scallion greens. Pass the remaining sauce and breadcrumbs at the table.