Step 1Heat the oven to 350 degrees. Place the turkey on a rack in a large roasting pan and season the cavity and outside generously with salt and pepper. Tie the turkey’s legs together with kitchen twine, if you like, then position the turkey breast side down on the rack and pour the stock in the pan. Cover the whole turkey and pan with a large sheet (at least 26 by 18 inches) of heavy-duty or double-thick foil and crimp it around the edges to seal. Bake, rotating the pan halfway through cooking, for 3 hours.
Step 2Meanwhile, make the glaze: Combine the maple syrup, sugar, mustard, chile flakes and lemon zest in a small saucepan, bring to a simmer over medium heat, and cook for 2 minutes. Remove from the heat and let cool.
Step 3Remove the turkey from the oven and increase the oven temperature to 400 degrees. Discard the foil and rotate the turkey so it’s breast side up. Return the turkey to the oven and roast, brushing it all over with the glaze every 5 minutes, until shiny and burnished, 40 to 45 minutes more. Remove the pan from the oven and transfer the turkey on its rack to a cutting board or rimmed baking sheet; let rest for 20 minutes before carving.
Step 4While the turkey rests, pour the pan drippings into a fat separator or tall glass and allow the fat to separate from the juices. Pour 2 cups of the juices into another measuring cup and discard the rest or save for another use. Spoon 2 tablespoons of the fat into a small saucepan and place it over medium heat. Add the flour and cook, whisking often, for 1 minute. Pour in the pan juices, whisking constantly, and cook until they thicken and create a smooth gravy. Season the gravy with salt and pepper and pour it into a warm bowl. Serve the gravy alongside the turkey.