Arrange bread in a single layer on a baking sheet and let stand overnight, uncovered, at room temperature. (Alternatively, bake bread in a 300 degrees oven until dry but not browned, about 15 minutes.)
Preheat oven to 375 degrees. Heat oil in a large saucepan over medium heat. Cook onion, carrot, and celery until tender and translucent, about 8 to 10 minutes. Transfer mixture to a large mixing bowl. Add remaining ingredients and toss to coat. Season with salt and pepper.
Transfer mixture to a large baking dish. Cover with foil and bake for 30 minutes. Uncover and continue baking until top is golden brown and stuffing is firm, about 15 minutes more.
Serve with Mushroom Gravy (optional).