Unwrap bird and remove contents out of the cavity if there is anything, discard. Place into stockpot and cover with water until it is completely submerged + about 2" above the top of the bird.
Slice one onion and add in with garlic (optional). Place on stovetop at medium high heat with a lid on. Check after about 15 minutes to ensure boiling is rolling consistently but isn't bubbling over.
Once it reaches a consistent boil it will take 1.5 hours in general, rule is; 1 1/2 hours to boil a 3 - 4 lb defrosted whole chicken. Larger than 4 pounds, add an additional 15 minutes per pound. For a frozen chicken add 15 minutes to the total cooking time as well.
It is done when you pull on the leg bone and it slides right out and internal temperature in thickest part of the meat reaches 165 degrees F.