Preheat the oven to 425 degrees. Line a baking sheet pan with foil sprayed with cooking spray. Slice the bread (or cornbread muffins) into 1/2-inch cubes. Toss the bread cubes with olive oil (or melted butter/oil of choice). Place on a baking sheet and toast in the oven for 15 minutes, tossing once, until the cubes are golden brown and slightly dry. Remove from oven and turn heat of oven down to 350 degrees.
Meanwhile, spray a large skillet with cooking spray, and heat over medium heat. Add garlic and onions and sauté 2-3 minutes, until tender. Add 1 cup of the chicken stock and bring to a boil. Add the celery, apple, seasonings, (and sausage if using), and sauté for 5-8 minutes, stirring occasionally, until vegetables and apple are just tender and all the stock is cooked off (evaporated). Remove from the heat.
Spray a 9x13 inch baking dish with cooking spray. Toss the bread cubes with the stuffing mixture, eggs, salt, pepper, and the rest of the chicken stock. Transfer the stuffing to the prepared baking dish. Cover and bake for 35 minutes. Uncover and bake for 15 minutes, or until the top is golden. Serve warm.