Preheat the oven to 425 degrees F. Remove the chicken giblets. Rinse the chicken inside and out. Remove any excess fat and leftover pin feathers and pat the outside dry.
In a mortar and pestle, combine 3 garlic cloves and ¼ teaspoon of kosher salt and pound into a paste. Next, add the allspice berries, grind them down to a paste, and transfer them to a small bowl.
Add the oregano, lemon zest, light brown sugar, chipotle powder, butter, paprika, Adobo seasoning, parsley, cilantro, and ground black pepper to taste and mash well to combine with a fork—reserve 3 tablespoons to the mixture to season the vegetables later.
Using your fingers, gently release the skin from the breast on either side of the breast bone. Place 2 tablespoons of the garlic & butter mixture under the skin on each side and smooth down to coat each breast. Do the same at the other end, where the thigh joins the open cavity and under the leg skin on each side.
Squeeze the remaining lemon into the chicken cavity, spread the cavity with the remaining garlic & butter mixture, and stuff with a bunch of thyme, both halves of juiced lemon, halved onion, and the head of garlic.
Rub olive oil over the chicken skin, including under the wings and legs. Then, sprinkle ½ tablespoon of the Adobo seasoning or kosher salt all over the chicken skin, including under the wings and legs, and try to distribute it evenly.
Tie the legs with the kitchen string and tuck the wing tips under the chicken's body. Place the onions, carrots, and potatoes in a roasting pan.
Add the vegetables, 5 sprigs of thyme, and drizzle the 4 tablespoons of olive oil and the reserved 3 tablespoons of the garlic mixture. Toss well to coat evenly, spread around the roasting pan's bottom, and place the chicken on top.
Cover the dish with plastic wrap and refrigerate for at least 4 hours or overnight. Remove the plastic wrap and bring it to room temperature for 30 minutes up to 1 hour before cooking—Preheat the oven to 425 degrees.
Roast for 1 ½ hours until the juices run clear when you cut between the leg and the thigh, and the internal temperature should be 165 degrees F on an instant-read thermometer. Remove the chicken and vegetables to a platter and cover with aluminum foil for about 20 minutes.
Cut off the twine. Pull the thigh and leg away from the breast of the chicken until the thigh bone "pops" out of the socket. Next, separate the thigh and drumstick.
Remove the breast from the carcass by feeling the ridge of the breastbone in the center of the roasted chicken and slicing around the rib cage. Arrange the roasted chicken on a serving platter or individual plates with the roasted vegetables.