Place all ingredients with the exception of the brown onion and ice cubes into a large stockpot. Bring to a boil over med/high- high heat until sugar and salt dissolves. Take off heat and add ice and brown onion. Let brine sit until ice has melted and brine has cooled to room temperature.
Open brining bag into a 36-42 quart stockpot and place turkey breast side down into bag. Pour brine over top of turkey and seal bag tightly following instructions on bag. Brine in refrigerator for 24-48 hours.
Once brined, remove turkey from liquid and dispose of brine. Throughly rinse off both the outside and inner cavity of bird and then pat dry with paper towels.
Preheat oven to 425℉.
Place turkey onto roasting rack inside inside large roasting pan. Add the onion, celery and garlic to the cavity of the turkey. Drizzle the outside of the turkey with olive oil and then sprinkle with salt and pepper.
Bake at 425℉ for 25 mins and then lower temperature to 350℉. Tent the turkey with aluminum foil (see below under Recipe Notes) and bake until internal temperature of the innermost part of the thigh reaches 165℉. If turkey has not acheived a golden brown color towards the end of roasting, remove foil for the last 30 mins of baking.
Once pulled from the oven let turkey rest for 30 mins before carving.