In a large roasting pan fitted with a rack, scatter most of the ginger and garlic head halves in the bottom of the pan. 

Place chicken on the rack and stuff cavity with remaining ginger, garlic cloves, green ends from 5 scallions, and 1 lemon half. 

Whisk together five spice powder with oil and rub all over chicken. Loosely tie the legs and set aside for about 30 minutes to marinate. 

Preheat the oven to 375°F. Roast chicken for 1 hour. 

After an hour, remove chicken from oven and add the remaining scallion to the bottom of the pan. Whisk together juice from remaining lemon half with soy sauce and pour over chicken. Roast for 30 minutes more or until thermometer inserted into the thickest part of the thighs reads 165°F. 

Remove chicken, vegetables, and lemon to a platter and cover with aluminum foil for about 10 minutes before carving. Once cool enough to handle squeeze lemon over chicken. Serve chicken with scallions and garlic, drizzled with pan drippings and pass any remaining pan drippings on the side.