dry mustard powder
low-sodium soy sauce
skinless, boneless chicken breast halves, about 6 ounce each
salt and freshly ground black pepper
medium head of bok choy, about 1 1/4 pound halved lengthwise
5-ounce bag baby arugula
1. Preheat the oven to 425°. In a bowl, stir the mustard with the water, mirin, 1 teaspoon of the soy and 1/2 teaspoon of the sugar.
2. In an ovenproof skillet, heat 1 tablespoon of the oil. Season the chicken with salt and pepper and cook over high heat until golden, 2 minutes. Flip the chicken and brush with the mustard; transfer to the oven and roast for 8 minutes, or until cooked through. Transfer the chicken to a cutting board and let rest for 5 minutes; slice. Wipe out the skillet.
3. Meanwhile, steam the bok choy for 5 minutes. Drain; pat dry. In the same skillet, heat 1 tablespoon of the oil. Add the bok choy cut side down and cook over high heat, turning once, until browned, 3 minutes. Transfer to a platter; season with salt and pepper. Arrange the chicken over the bok choy.
4. In a bowl, whisk the vinegar with the remaining oil, soy sauce and sugar; season with salt and pepper. Add the arugula and toss; arrange over the chicken and serve.