Use paper towels to pat chicken as dry as possible, both inside the cavity and the exterior skin.
Liberally salt and pepper the inside of the chicken.
Stuff the cavity with the bunch of thyme, both halves of lemon, and all the garlic.
Brush the outside of the chicken with the butter and sprinkle again with salt and pepper.
Truss the bird.
Place the onions, yams, and fennel in a roasting pan.
Toss with salt, pepper, 20 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 1/2 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 20 minutes. Slice the chicken onto a platter and serve it with the vegetables.
Notes
Patting the chicken dry is essential to attaining a crispy outside. After 20 minutes of roasting, drizzle/brush a little melted over the chicken. Monitor for oven hot spots, rotate roasting pan a few times if necessary.