Fragrant spices such as coriander seeds, bay leaves, juniper berries, fennel seeds, and peppercorns permeate the buttermilk brine for this holiday centerpiece. It's an easy way to create big flavor with your bird this Thanksgiving.
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.