1. Preheat oven to 350 degrees, with rack in center. Place the garlic head on a square of aluminum foil, and drizzle with oil. Loosely wrap garlic, and roast until fragrant and very soft, about 1 hour. Remove from oven; let cool.

2. In a small bowl, combine herbs and butter. Slice off tip of roasted garlic head with a serrated knife; discard. Squeeze garlic head firmly over bowl to release softened cloves. Season with salt and pepper, and mix until smooth.

3. Raise oven temperature to 400 degrees. Rinse turkey with cool water; pat dry. Tuck wing tips under body. Place turkey on a roasting rack in a heavy roasting pan; add 1/2 cup water to pan.

4. Using your hands, gently separate skin from turkey breast at both ends; smear one-fourth to one-third of herb butter between skin and meat. Smear exterior and cavity with remaining herb butter.

5. Roast until skin begins to brown, 50 to 55 minutes (tent with foil if herbs begin to burn). Reduce oven temperature to 350 degrees; continue to roast turkey, basting with additional butter, until a leg rotates easily in the joint, 70 to 90 minutes more (depending on size of turkey). Insert an instant-read thermometer into thickest part of thigh (avoid the bone) to see if temperature has reached 180 degrees; if not, baste turkey, return to oven, and roast until temperature is 180 degrees. Let turkey rest, loosely covered with foil, at least 30 minutes before carving. Arrange turkey on a platter with onions, and serve.