In a heavy roasting pan or very large ovenproof skillet, heat the oil over medium heat. Add the onions, celery, carrot, the remaining rosemary and sage, and the bay leaves, and cook, stirring, until the onions are wilted, about 6 minutes. Smooth the vegetables into an even layer, and nestle the prepared hens, breast side up, over them. Roast, basting with enough of the chicken stock to keep the vegetables well moistened, until the vegetables and hens are golden brown, about 1 hour. The leg joint should wiggle somewhat freely.