Cut onion into large pieces. Separate garlic into cloves, remove protective skin and crush.
Bring large pot to boil with all ingredients. After full boil, let simmer 5 minutes.
After simmering, fill the pot the rest of the way with ice. Refrigerate until cool. Brine MUST be cold before use on the turkey.
Remove turkey from packaging and remove giblet pack. Rinse bird inside and out and pat dry. Turkey neck will be in the chest cavity, the giblet pack will be in either the chest or neck cavity.
Set up brine bag in large baking pan or for smaller turkey in a crock pot. This will help prevent from getting turkey brine or blood everywhere.
Put turkey in brine bag and cover turkey with ice. Pour the brine solution over the turkey and tie a secure knot. Turkey must be completely submerged. If you need more liquid, add water.
Let turkey in brine sit for up to 24 hours.