Fragrant spices permeate the buttermilk brine for this holiday centerpiece.
Bring 1 quart water, salt, bay leaves, and spices to a simmer in a medium saucepan, stirring until salt has dissolved. Let cool 5 minutes.
Line a 5-gallon container with a large brining bag. Place turkey in bag with 4 quarts water, salt mixture, buttermilk, onions, garlic, and thyme; tie bag. (If turkey is not submerged, weight it down with a plate.) Refrigerate for 24 hours, flipping turkey once.
Remove turkey from brine; discard brine. Rinse turkey and pat dry. Let stand at room temperature 1 hour.
Preheat oven to 400 degrees. Tie turkey legs and tuck wing tips under. Transfer to a large roasting pan fitted with a rack. Place in oven, legs first, if oven permits. Roast 45 minutes, then lower temperature to 325 degrees and roast, rotating and basting about every 30 minutes, until a thermometer inserted into thickest part of thigh (avoiding bone) registers 165 degrees, 2 to 2 1/2 hours. Let stand at least 30 minutes before carving.