Preheat your oven to 350°F (175°C).
In a skillet, heat olive oil over medium heat. Add onions, celery, and garlic. Sauté until softened.
Add diced Portobello mushrooms to the skillet. Cook until they release their moisture and start to brown.
In a large bowl, combine the sautéed vegetables, bread cubes, thyme, sage, salt, and pepper. Mix well.
Gradually add vegetable broth to the mixture, stirring until the bread is moistened but not soggy.
Transfer the mixture to a baking dish. Cover with foil and bake for 30 minutes. Remove foil and bake for an additional 15 minutes until the top is golden brown.
Serve hot.