Preheat oven to 425 degrees. In a large bowl, toss together potatoes, carrots, onion, oil, thyme, salt and pepper. Line a baking sheet with tinfoil and spread out the vegetables onto the pan. Roast vegetables on pan for 15 minutes.
Place chicken in a single layer over vegetables. Season with salt and pepper as desired. Roast for another 15 minutes.
In a medium bowl, whisk together Dijon mustard, soy sauce, honey, whole-grain mustard, and brown sugar. Drizzle over chicken and vegetables. Roast for another 25 minutes until chicken is cooked (165 degrees) and vegetables are tender and golden brown. Serves 4-6.