How to Make It
Preheat oven to 475°F. Coat a large roasting pan with cooking spray. Trim chicken of excess fat. Place lemon halves inside cavity. Truss or tie legs with kitchen string; tuck wings underneath body. Set chicken in center of roasting pan and rub all over with butter. Season generously with salt and pepper.
In a large bowl, toss leeks, carrots, parsnips, turnips, potatoes, shallots and thyme with olive oil. Season with salt and pepper. Arrange vegetables around chicken; pour in broth. Roast for 25 minutes, then reduce oven temperature to 400°F. Cook until an instant-read thermometer inserted into thigh reads 165°F, 50 to 60 minutes.
Transfer chicken to a large platter, cover loosely with foil and let rest for 10 to 15 minutes. Before serving, remove any excess fat from roasting pan, then place pan, with vegetables and pan juices, over medium-high heat. (If there are not enough pan juices, pour in 1/4 cup chicken broth.) Cook, tossing vegetables, until warmed through, shiny and glazed, 3 to 5 minutes. Uncover chicken, surround with vegetables and serve warm.