Preparation Whisk sugar, mirin, and miso in a medium bowl to blend; add fish and turn to coat. Cover and chill, turning fish occasionally, at least 12 hours and up to 1 day. Heat 1 tablespoon oil in a medium nonstick skillet over medium heat. Remove fish from marinade, scraping off excess; season with salt and pepper. Cook fish skin side down until dark brown and caramelized, about 4 minutes. Turn fish over and cook until just opaque in the center, about 3 minutes longer. Meanwhile, heat remaining 1/2 tablespoons oil in a medium skillet over medium-high heat. Add greens and cook, tossing, just until wilted, about 1 minute. Season with salt and pepper; divide between plates. Add vinegar to same skillet and cook, stirring constantly, until thickened, about 30 seconds; drizzle reduced vinegar over greens. Top with fish.
Preparation Whisk sugar, mirin, and miso in a medium bowl to blend; add fish and turn to coat. Cover and chill, turning fish occasionally, at least 12 hours and up to 1 day. Heat 1 tablespoon oil in a medium nonstick skillet over medium heat. Remove fish from marinade, scraping off excess; season with salt and pepper. Cook fish skin side down until dark brown and caramelized, about 4 minutes. Turn fish over and cook until just opaque in the center, about 3 minutes longer. Meanwhile, heat remaining 1/2 tablespoons oil in a medium skillet over medium-high heat. Add greens and cook, tossing, just until wilted, about 1 minute. Season with salt and pepper; divide between plates. Add vinegar to same skillet and cook, stirring constantly, until thickened, about 30 seconds; drizzle reduced vinegar over greens. Top with fish.