Preheat oven to 350 degrees. Lightly grease a 2 quart casserole dish. Place the croissant pieces in a large bowl.

In a large skillet over medium heat, melt the butter. Add the celery, onion, mushrooms, garlic and ground sage and cook until the vegetables are tender, 6-8 minutes.

Add the vegetable mixture to the croissants in the bowl, along with the fresh chopped herbs and toss to mix well.

In a small bowl, whisk together the broth and egg. Gradually add the broth mixture to the stuffing until everything is well moistened (you may not use all the broth). Season the stuffing with salt and pepper, to taste.

Transfer the stuffing to the prepared casserole dish and bake, uncovered, for about 30 minutes, or until stuffing begins to turn golden brown on the edges and is hot in the center.