Preheat the oven to 400 degrees F. Put the diced celeriac, carrots, and leek in a large roasting pan and drizzle with vegetable oil. Season the vegetables with salt and pepper, then toss and spread evenly in pan. Season the inside of the chicken with salt and pepper. Stuff the chicken, beginning with 1/2 lemon, shallots, thyme, sage, and garlic, and ending with the other 1/2 lemon. Tuck the wings under the chicken to secure and truss the chicken legs with kitchen twine. Season the outside of the chicken well with salt and pepper and place the bird on top of the oiled vegetables. Place the roasting pan in the oven and roast until the chicken is cooked through and the juices run clear, approximately 1 1/2 hours or when an instant read thermometer inserted in the thigh, not touching the bone, reaches 185 degrees F.