Well I cheated and found an online recipe to paste the link here but I don't put potatoes or celery in my beef & Guinness stew. I also use a different Aussie brand of stout which is called "Cooper's extra stout" because Guinness is more expensive. For thickening, near the end of the cooking time I put about 3/4 cup of red lentils in and cook it for the last 20 minutes. It thickens the stew nicely, the flavour is not over-powering, it blends in well with the flavour of the stew, and it has the advantage of adding a bit of healthy fibre.

The beef in red wine is a great variation too and is a very similar recipe to the beef & Guinness.