Preheat oven to 350 degrees F (175 degrees C). Grease a glass baking dish with cooking spray.
Heat the olive oil in a large skillet. Cook the mushrooms, garlic, celery, and onion in the olive oil 3 to 5 minutes. Add the water and bouillon; cook and stir until the bouillon is dissolved.
Combine the garbanzo beans, egg, sage, salt, and pepper in a bowl. Mash together with your hands until thoroughly mixed; stir in the vegetable mixture; transfer to the prepared baking dish. Cover with aluminum foil.
Bake in preheated oven 30 minutes; remove the foil and bake uncovered another 10 to 15 minutes.
