Preheat oven to 350°F (175°C).
In a large skillet, heat olive oil over medium heat.
Add onion, celery, and garlic. Cook until softened.
In a large bowl, combine bread cubes, cooked vegetables, parsley, sage, thyme, salt, and pepper.
Pour vegetable broth over the mixture and toss until well combined.
Transfer to a baking dish and cover with foil.
Bake for 30 minutes, then remove foil and bake for another 15 minutes until golden brown.