Adapted from Anne Quatrano's "Summerland: Recipes for Celebrating with Southern Hospitality" (Rizzoli).
Fill a large stockpot or food-grade bucket three-quarters full with warm water. Add brown sugar and 2 cups salt, stirring until dissolved. Add onions, celery, garlic, thyme, bay leaves, coriander, peppercorns, and fennel seeds. Let cool to room temperature. Place turkey in brine. Add cool water, if necessary, to fully submerge turkey. Cover and refrigerate 24 hours.
Remove turkey from brine. Rinse inside and out; pat dry with paper towels. Season cavity with salt and pepper. Rub room-temperature butter evenly over outside of turkey; season with salt and pepper. Tuck wings under turkey; tie drumsticks together with kitchen twine. Transfer to a roasting pan fitted with a rack. Let stand 1 hour.
Preheat oven to 425 degrees with rack in lower third. Roast turkey 30 minutes. Baste with some of melted butter; lower oven temperature to 350 degrees. Continue roasting, basting with butter every 30 minutes, until a thermometer inserted into thickest part of breast near (but not touching) bone registers 165 degrees, 1 to 1 1/2 hours total, depending on size of bird.
Transfer turkey to a serving platter. Reserve pan with drippings for gravy. Let turkey rest at least 45 minutes; carve and serve with gravy and garnishes.
Basting the turkey with melted butter as it roasts ensures crisp, evenly browned skin.