1. In a very large (14- to 16-quart) stockpot, combine the salt, sugar, peppercorns, star anise, cinnamon, ginger, garlic, chile flakes, 1 1/2 cups syrup, and 1 cup soy sauce. Heat over medium heat, stirring continuously, just until the salt and sugar dissolve. Remove from the heat and stir in 5 quarts water. Let the mixture stand until room temperature.
2. Submerge the turkey in the liquid, press a piece of plastic wrap over the turkey and weight the bird down with a heavy dish of small pot so that the turkey stays in the liquid. Refrigerate for 1 to 3 days.
3. Remove the turkey from the brine, pat dry with paper towels, and discard the brine. Place the turkey on a rack set in a large roasting pan. Let stand for 1 hour. Preheat the oven to 425 degrees F.
4. Tuck the wing tips behind the back and tie together the ends of the drumsticks with kitchen twine. Scatter the onions around the turkey. Season all lightly with salt and pepper.
5. Roast the turkey for 30 minutes, then reduce the oven temperature to 325 degrees F. Add the remaining 2 cups water to the pan. Continue roasting for 1 1/2 to 2 hours or until a meat thermometer inserted into the thickest part of the thigh registers 165 degrees F. Baste the turkey two or three times during the roasting.
6. Transfer the turkey and onions to a serving platter and let rest while making the sauce. Strain the pan juices into a fat separator. Pour just the jus back into the roasting pan; discard the fat. Set over medium heat and stir in the remaining 2 tablespoons syrup and 2 tablespoon soy sauce. Simmer, stirring and scraping the pan, until reduced by one-third, about 5 minutes. Serve the jus with the turkey and onions.