Remove the pack of giblets from the chicken. Save for use in homemade stock, if desired. Place the chicken in a roasting pan. (I often use a 9×13 Pyrex baking dish.) Put the onion and lemon halves in the cavity of the chicken and sprinkle salt, pepper, and thyme all over the inside and outside.
Bake, uncovered, at 275 degrees F for 3-4 hours. If chicken is smaller, cook on the low end of the time frame and vice-versa. But really, if you keep the chicken in a half hour longer than intended, it will still taste great. I’ve done this!
Let the chicken rest for 10-15 minutes before carving.