Whole Roasted Lemon-Herb Chicken on a Bed of Vegetables

Position rack in lower third of oven; preheat to 400ºF. Chop 8 sage leaves and place in a bowl with 2 teaspoons thyme leaves. Squeeze the juice from 1 lemon into the bowl. Add garlic, 1 tablespoon oil, 3/4 teaspoon salt and 1/2 teaspoon pepper; mix well. Set aside. Pierce the remaining lemon all over with a sharp fork. Cram the lemon, onion and the remaining sage and thyme into the chicken cavity. Place the chicken breast-side up on a cutting board. Use your hands to gently loosen the skin covering the breast, thighs and the top end of the drumsticks. Smear the lemon-herb mixture under the skin, covering as much of the meat as possible. Toss carrots, turnips and celeriac in a large bowl with the remaining 1 tablespoon oil and 1/4 teaspoon each salt and pepper until well coated. Place the chicken breast-side up in a large roasting pan (but not on a rack). Scatter the vegetables around the chicken. Bake, stirring the vegetables occasionally, until an instant-read thermometer inserted into the thickest part of a thigh without touching bone registers 165°F, 1 to 1 1/4 hours. Transfer the chicken to a clean cutting board and let rest for 10 minutes before carving. Transfer the vegetables from the roasting pan to a serving dish with a slotted spoon, leaving behind as much of the fat as possible. Serve the chicken with the vegetables.