In a frying pan, cook and stir celery, onion, mushrooms and garlic for 3 minutes. Stir in 1/3 of the chicken stock and cook until vegetables are tender. Combine mushroom mixture with stuffing mix, and toss to coat.
In a 2 litre saucepan, heat remaining chicken stock until hot. Pour stock over mixture. Stir well.
Transfer stuffing into a 2 litre casserole or baking dish and bake for 30 to 40 minutes.