Whisk together soy sauce, brown sugar, sesame oil, ginger, and garlic in a very large bowl.
Place turkey breast side down into bowl. Marinate at room temperature for 30 minutes. Turn turkey and marinate breast side up for 15 minutes. Turn turkey breast side down and marinate for another 15 minutes.
Preheat oven to 350 degrees F. Remove turkey from marinade. Reserve marinade. Place turkey, breast side up*, on a rack in a roasting pan. Season turkey inside and out with salt and pepper. Stuff the turkey cavity with scallions, garlic, and onion.
Loosely cover turkey with foil tent. Place in oven. Pour 2 cups water into roasting pan. Roast turkey for 4 1/2 hours, basting the turkey with reserved marinade every hour. After each basting, add 1/2 cup of water to roasting pan.
Remove foil tent. Roast uncovered for 30 minutes, basting once more after 15 minutes. Turkey is cooked once the an instant-read thermometer inserted in the meaty part of the thigh registers 165 degrees F. If turkey is not done, add foil tent and continue to bake until fully cooked.
Transfer turkey to a cutting board or large serving plate. Let stand for at least 45 minutes before slicing. Before serving, garnish turkey with toasted sesame seeds. Meanwhile, make gravy.