Preheat the oven to 325° F. Spray a 9 x 13-inch casserole dish with cooking spray.
Combine the onions, celery, tarragon, and parsley in the prepared casserole dish.
Rub the chicken pieces with the olive oil and season with the salt and pepper. Put the chicken on top of the onions and celery. Drizzle with the vermouth and tuck the garlic in and around the chicken.
Cover the casserole with foil. Bake for 1½ hours, or until internal temperature of the chicken is 160° F.