Place the butter in a small, light-colored skillet over medium-low heat.
Allow the butter to melt, then bubble and boil.
Continue to cook the butter, swirling the pan occasionally, as it bubbles down to a simmer.
Watch carefully as the butter becomes foamy and the milk solids begin to brown.
When the butter takes on a nutty, amber color, toss in the sage leaves (be careful, they will sizzle and spit).
Immediately pour the sage brown butter from the skillet into a heat-safe bowl to halt the cooking process, and vigorously whisk in the maple syrup, salt, and pepper.
To Cook the Turkey
Preheat the oven to 325 degrees F.
Remove the giblets from both cavities of the turkey, tuck the wing tips behind the bird, and tie the ankles together with kitchen string.
Place the bird in a roasting pan, and brush it with about half the glaze.
Cook until the juices run clear (not pink) and the thigh meat registers an internal temperature of 165 degrees F, basting with the glaze every 30 minutes (approx. 2 1/2 to 3 hours total for an 18-pound turkey).*
Allow the turkey to rest for 1 hour at room temperature before carving.
Notes
*If at any point the bird is starting to become too brown, cover that area loosely with foil.