Mince 1 sprig rosemary to total 2 teaspoons. Cut 1/2 red onion into 1/2 in. thick slices. In large bowl, combine potatoes, carrots, red onion, minced rosemary, garlic, 1 teaspoon salt, 1/2 teaspoon pepper and 2 tablespoons oil; toss to coat. Place vegetables in medium roasting pan. Squeeze 1 lemon half over vegetables.
Rub chicken with remaining 1 tablespoon olive oil. Season inside and outside of chicken with 1 teaspoon salt and 1/2 teaspoon pepper. Place remaining 1/2 red onion, 1/2 lemon and 2 sprigs rosemary inside chicken cavity. Tuck wings under chicken and tie legs together with kitchen twine. Place on top of vegetables, breast side up.
Bake 20 minutes, then reduce oven temperature to 425°F. Bake 45-55 minutes longer or until instant thermometer inserted into the thickest part of the thigh reads 165°F. Remove to cooling grid; cover with aluminum foil 5-10 minutes. Serve warm.