Preheat the oven to 425 degrees F.
Carefully rinse the chicken inside and out. Pat dry inside and out, too. Liberally salt and pepper the cavity of the chicken. Stuff with the bunch of thyme, both halves of lemon, and all the garlic. Brush the outside of the chicken with the butter and dust with salt and pepper. Tie the legs together with kitchen string and tuck the wing tips under the body of the chicken. Place the onions, carrots, and fennel in a roasting pan. Toss with salt, pepper, 15 sprigs of thyme, and olive oil. Spread around the bottom of the roasting pan and place the chicken on top.
Roast the chicken for 1 1/4 hours, or until the juices run clear when you cut between a leg and thigh. Remove the chicken and vegetables to a platter and cover with aluminum foil for about 15 minutes. Carve the chicken and serve it with the vegetables.