Heat the oil in a large saucepan over a medium heat, add the onion and garlic and cook until soft but not coloured. Add the celery, mushrooms and tomato purée and cook until they, too, are soft, then add all the remaining vegetables and mix.
Pour in the Guinness and stock along with the rosemary and thyme and leave to simmer for 1 hour, until all the veg are tender.
Mix the cornflour in a small bowl with a little water to make a loose paste and add to the mixture, then simmer for a while to allow the stew to thicken. Season with salt and pepper to taste. You won’t need anything on the side with this: everything you need is there!