Preheat oven to 425°F.
Pat outside of chicken dry, then liberally season the inside with salt and pepper.
Stuff thyme, lemon, and garlic into the cavity.
Brush the outside of the chicken with melted butter and season all over with salt and pepper.
Tie the legs together with kitchen twine and tuck the wing tips under the body (so they don't overcook).
Place onions, carrots, and fennel in a roasting pan and toss with salt, pepper, thyme, and olive oil.
Place chicken on top of veggies and roast for 1 ½ hours.
Remove the chicken and cover with foil and let it rest for 20 minutes.
