Remove chicken from the refrigerator 20 minutes before roasting. Place a 12-in. cast-iron skillet in the lower third of oven; preheat oven to 450°.

Meanwhile, place beets, carrots, parsnips and radishes in a large bowl. Drizzle with 1 Tbsp. olive oil; sprinkle with ¼ tsp. salt and ¼ tsp. black pepper. Toss to coat; set aside.

Cut 1 lemon into wedges; set aside. Zest and juice remaining lemon. Whisk together lemon zest and juice, 2 Tbsp. tarragon, gingerroot, remaining 1 Tbsp. olive oil, garlic, ¼ tsp. salt and ½ tsp. black pepper in a small bowl. Set aside.

Pat inside and outside of chicken completely dry with paper towels. Place chicken, breast side up, on work service. Gently slide your fingers under the breast skin to loosen the skin, being careful not to tear the skin.

Rub lemon-ginger mixture between meat and skin. Rub any remaining lemon-ginger mixture in cavity of chicken. Rub remaining ½ tsp. salt and ½ tsp. pepper on outside of chicken. Pierce breast skin with the tip of a sharp knife in a few places to prevent skin from bubbling up. Place reserved lemon wedges in cavity of chicken. Tie drumsticks together with kitchen string. Twist wing tips under back.

Remove cast-iron skillet from oven. Spread vegetable mixture evenly in skillet. Place chicken, breast side up, on top of vegetable mixture.

Roast chicken for 15 to 20 minutes. Add chicken stock to bottom of skillet. Reduce oven temperature to 350°. Roast for 1 to 1½ hours or until chicken reaches 165° in thickest part of thigh. If necessary loosely cover with foil during the last 40 minutes of roasting to prevent overbrowning. Transfer chicken to a cutting board; loosely cover with foil and let rest.

Meanwhile, tightly cover skillet with foil. Roast vegetables in cooking juices for 6 to 8 minutes more or until fork-tender. Garnish with additional tarragon, if desired. Serve cooking juices with chicken and vegetables, if desired.