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.
Video
Notes
Boiled Whole Frozen Chicken
If yours is rock hard you will likely need to add about 15 minutes to the cook time. Halfway thru you could carefully use tongs to lift the neck area out to remove the giblet bag if there is one present.Before slicing test the thickest part of the breast or thigh to ensure it has reached a safe 165F internally.