Carefully debone the legs and thighs while leaving them attached to the chicken.

Fill a large pot with 5 quarts water and bring it to a boil over high heat. Turn off the heat. Add the salt, brown sugar, bay leaves, and peppercorns and stir until the salt and sugar are fully dissolved. Set the brine aside to cool completely.

Once the brine has cooled, put the chicken in the pot and set a heavy plate on top of it to keep it submerged. Cover the pot with plastic wrap, put it in the refrigerator, and let it sit for 8 to 12 hours.

Pull the chicken out of the brine, rinse it under cold running water, and pat it dry with paper towels. Discard the brine. Leave the chicken out until it comes to room temperature, about 15 minutes.

Adjust the oven rack to the middle position and preheat the oven to 425°F.

Using your fingers to separate the skin from the meat, slide a lemon slice, a few garlic slices, and 1 teaspoon of the parsley leaves under the skin of each breast. Put the remaining lemon slices, garlic slices, and parsley leaves in the chicken cavity. Close the cavity by crossing the legs and securing them together with a wooden or metal skewer.

Brush the chicken with the olive oil. Cut each celery stalk in half crosswise. In a roasting pan, arrange the celery pieces in a line like ties on a railroad track (you’re creating a base to prevent the chicken from sticking to the pan). Lay the chicken right on top of the celery. Roast the chicken for 20 minutes, until the skin starts to develop a tan color. Then reduce the oven temperature to 350°F and roast for another 50 to 60 minutes, until its internal temperature is 165°F. (Use a meat thermometer to take the temperature, inserting it between the breast and the thigh and making sure it isn’t touching the bone.) Allow the chicken to rest for 10 minutes before carving.

To split the chicken in half, separate the breasts by making a long cut along the spine with a sharp knife to reveal the bone. Carefully carve each breast off the carcass and cut the joint that keeps the wing bone attached, while keeping the breast connected to the leg and thigh. Transfer to a serving platter or two separate plates, and serve immediately.