Preheat the oven to 400ºF.
Sprinkle the butter slices with 1/2 teaspoon of the salt and 1/4 teaspoon of the cayenne. Freeze the butter for about 15 minutes, or until firm.
Season the onions, celery, bell peppers, and garlic with 1/2 teaspoon of the salt, the remaining 1/4 teaspoon cayenne, the dried thyme, and the juice of one lemon (about 2 tablespoons). Reserve the lemon halves.
With the chicken breast side up, and the neck opening facing you, lift the skin flap and make a slit, about 1 inch long, down the wing from the shoulder following the bone line, on each side of the neck, using a sharp pointed knife. Fill each slit with 2 slices of butter and 1 to 2 tablespoons of the vegetables. Press the butter and vegetables into each slit with your fingers.
Fold the neck flap down and turn the chicken, breast side up, with the cavity facing you. Make several slits, each about 1 inch long, inside the cavity along the breastbone and rib bones. Do not pierce through to the skin. Insert the butter slices and vegetables in each slit. Again push in the butter and vegetables with your fingers. Put any remaining butter and vegetables in the cavity. Rub the chicken with the lemon halves, then put them and the sprigs of thyme inside the cavity. Season the outside of the chicken with the remaining salt and the black pepper.
Place the chicken, breast side up, in a roasting pan and roast, uncovered, for about 45 minutes, or until golden brown. Place a sheet of aluminum foil loosely over the chicken to cover the drumsticks and breast. Roast for 45 minutes to 1 hour more, or until the chicken is tender and the drumsticks and thighs are easy to move. Baste occasionally with the pan juices during the last 30 minutes of roasting. Remove foil.
Remove the thyme sprigs and lemon halves. Allow the chicken to sit for 15 minutes and then carve. Lay the cut-up pieces in the pan juices for a few minutes and serve.
Yield: 4 to 6 servings