Preheat the oven to 350 degrees F. Lay the sliced bread onto a baking tray in a single layer. Bake the bread in the oven for 5 minutes to dry out the bread. You can also leave the bread out overnight to dry it out.
While the bread is baking, preheat a large skillet over medium heat. Add the olive oil and butter. Once hot, add the celery, carrots, onion, salt and pepper and saute until the vegetables have softened, but have not browned. This should take 5-7 minutes.
Meanwhile, slice the bread into small cubes and then add them to the skillet along with the sage and parsley. Mix everything together to combine.
Once mixed, remove the stuffing from the heat and add the stuffing into a 9x9 inch baking dish. Spread the stuffing evenly in the dish then pour the chicken stock all over the stuffing to moisten it.
Cover the baking dish with aluminum foil and bake in the oven for 1 hour. Once the hour has elapsed, remove the aluminum foil from the baking dish and continue to bake for an additional 15-20 minutes so that the top gets nice and golden brown.
Remove the stuffing from the oven and let cool for 10 minutes before serving. Enjoy!