Heat oven to 400° F.
Pat the chicken dry with paper towels. Combine the mustard, soy sauce, and 1/4 teaspoon pepper in a large bowl. Add the chicken and stir to coat.
Combine the carrots, fennel, onion, thyme, oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a roasting pan.
Nestle the chicken among the vegetables.
Roast until the chicken is cooked through and the vegetables are tender, 45 to 50 minutes.
Divide the chicken and vegetables among individual plates.