
In a large skillet, melt the butter over medium heat. Add the celery, onion, salt, and pepper and cook for 4 minutes. Add the mushrooms and garlic and cook for another 4 minutes, stirring occasionally. Remove the vegetables from the heat.
Meanwhile, in the base of a 6-quart slow cooker, whisk to combine the soup, broth, egg, parsley, rosemary, and sage. Add the bread and fold together until the cubes are well coated. Gently fold in the vegetable mixture.
Cover and cook on high heat for 30 minutes. Then reduce the heat to low for 2 more hours. (Do not open or stir while cooking.) The top of the stuffing should be steaming and the temperature in the center should be around 160°F when finished cooking. Transfer to a serving bowl, or hold on warm heat in the slow cooker, covered, for up to 1 hour.