Preheat the oven to 350 degrees F. Spray a 9 by 13-inch baking dish with cooking spray.
Remove the mushroom caps from the stems. Reserve the caps for the online round 2 recipe Stuffed Mushrooms. Roughly chop the stems and set aside.
In a large skillet over medium-high heat, add the canola oil. Stir in the onions and celery and cook until softened slightly, about 2 minutes. Add the mushroom stems and herbs and continue to cook until the mushrooms are browned and the vegetables are slightly tender, about 5 minutes.
Whisk together the eggs and chicken broth in a large bowl. Add the cubed bread and toss to coat all of the bread. Stir in the cooked onions and mushroom mixture and toss to combine. Season with salt and pepper, to taste. Transfer to the baking dish and bake, covered loosely with foil, for 40 to 50 minutes. Remove the foil for the last 10 minutes of cooking time to brown the top. Remove from the oven and serve.
Use the leftovers from this recipe to make quesadillas and stuffed mushrooms.