Preheat the oven to 400 degrees. Remove and discard all the stuff inside the chicken. Rinse the chicken inside and out and pat dry. Stuff the cavity with the thyme, half the lemon, and the garlic cloves. Brush or rub the outside of the chicken with the olive oil and sprinkle generously with salt and pepper. Roast the chicken breast side down in the pan for the first 20 minutes, then remove. Turn oven down to 350 degrees. Place the onions, carrots, fennel and potatoes in the roasting pan. Toss with olive oil, salt and pepper. Spread around the roasting pan and then place the chicken back on top with the breast up. Roast the chicken for another hour, or until the juices run clear when you pierce the leg or thigh with a knife. Remove the chicken and vegetables to a platter and cover with aluminum foil for about 10 minutes so all the juices settle. Slice the chicken onto a platter and serve it with the vegetables.