Preheat oven to 180 degrees C. Grease a glass baking dish with cooking spray.
Heat the olive oil in a large frypan. Cook the mushrooms, garlic, celery and onion in the olive oil 3 to 5 minutes. Add the water and stock cubes; cook and stir until the stock is dissolved.
Combine the chickpeas, 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 aluminium foil.
Cook in preheated oven 30 minutes; remove the foil and cook uncovered another 10 to 15 minutes.