Adjust oven rack to upper middle position a dn heat oven to 475 degrees. Mean while mince garlic and rosemary. Combine 3 tablespoons mustard, 1 tablespoon oil, soy sauce, garlic, rosemary and 1/8 teaspoon pepper in small bowl.
Trim chicken and cut breasts in half crosswise, pat dry with paper towels adn season with salt and pepper. Place chicken, skin side up, on rimmed baking sheet, arranging breast pieces in center and leg and thighs on edges. Brush chicken with mustard mixture.
Roast chicken for about 25-30 minutes, rotating pan halfway through cooking.
While chicken roasts, bring 2 quarts water to boil in large saucepan over medium high heat. halve potatoes. Trim green beans and cut into 1 1/2 inch pieces. Add potatoes and 1 1/2 tablespoons salt to boiling water, return to boil and cook for ten minutes. Add green beans and cook until bot vegetables are tender, about 5 minutes. Drain well and return pot to keep warm.
While vegetables cook, chop 2 tablespoons parsley, rinse and mince capers. Whisk vinegar, remaining 1 teaspoon mustard and capers together in large bowl. Whisking constantly, slowly drizzle in remaining 3 tablespoons oil unitl incorporated.
Transfer chicken to platter. Scrape up any browned bits and pour any accumulated juices from baking sheet into dreassing. Add warmedvegetablesto bowl with dressing and toss gently to combine. Sprinkle chicken and salad with parsley.