Heat oven to 450 degrees. In a small bowl, combine scallions, mustard, ginger, garlic, miso, sugar, 1 teaspoon salt, 1⁄2 teaspoon pepper and 6 tablespoons of the oil. Mix well.

On a rimmed baking sheet, toss bok choy with the remaining 2 tablespoons oil and season with salt and pepper, then spread in an even layer. Season chicken with salt and pepper, arrange on top of bok choy and rub chicken all over with the scallion-mustard marinade.

Roast until chicken is cooked through and vegetables are tender, about 30 minutes.

Serve chicken and vegetables over rice or mashed potatoes. Spoon pan juices over the chicken and garnish with scallions. Serve with lemon wedges.