Combine two litres water, salt, maple syrup, black peppercorns, mustard seeds, and thyme, chili flakes in a large stockpot over medium-high heat. Stir occasionally to dissolve solids and bring to a boil. Then remove the brine from the heat, cool to room temperature, and refrigerate.
Combine the brine, four litres of water, two litres of ice and beer. Using a washed cooler or container, place the turkey (with innards removed) breast side down in brine. If necessary, weigh down the bird to ensure it is fully immersed, cover, and refrigerate or set in cool area for 8 to 16 hours, turning the bird once half way through brining.
Preheat the oven to 350F*. Remove the bird from brine and rinse inside and out with cold water. Discard the brine.
Place the bird on roasting rack inside a half sheet pan and pat dry with paper towels.
Combine the apple, onion, cinnamon stick, and one cup of water to a small pot and bring to a boil. Add steeped aromatics to the turkey's cavity along with the rosemary, thyme, and bay leaves Tuck the wings underneath the bird and coat the skin liberally with olive oil.
Roast the turkey on lowest level of the oven at 350* F for 2-1/2 to 3 hours. Insert a probe thermometer into thickest part of the breast looking for a internal temperature of 160*F. Let the turkey rest, loosely covered with foil for 15 minutes before carving.