Preheat the oven to 425 degrees F.
Rinse the chicken well inside and out with water and pat dry with paper towels. Rub the chicken all over with the olive oil and season inside and out with salt and pepper. Sprinkle the thyme and rosemary all over the chicken and place the reserved stems inside the chicken cavity. Arrange the onion slices, carrots, celery and bay leaf in the bottom of a small roasting pan in such a way as to make a bed for the chicken to lie on. Place the chicken on the bed of vegetables, breast side up, and roast in the oven until the skin is golden brown, 30 to 45 minutes. Reduce the oven temperature to 375 degrees F and continue to roast until the chicken is cooked through and the juices run clear, 30 to 45 minutes longer.
Remove the chicken from the oven and set aside to rest for 10 minutes before carving.
When ready to serve the chicken, carve into 8 pieces and serve, drizzled with truffle oil to taste. Garnish with fresh herb sprigs.