Place the mushrooms in a microwave-safe bowl and cook on high heat for 3 minutes to release the water. Strain and set aside.
Heat 3 tablespoons oil in a large pan on medium-high heat. Start by browning the beef in batches on all sides. Add to the slow cooker.
Using the same pan, add more oil if needed. Soften onion and celery. Add the garlic and cook another minute.
Stir in the beer. Crumble in the stock cube and add the mushrooms and thyme. Bring to a boil, then add the mixture to the slow cooker with the beef. Season well with salt and pepper. Cook on low 8-10 hours.
Before serving the remove the lid. Add the flour to the water and mix until no lumps appear in the mixture. Mix into the stew and cook uncovered for 30 more minutes on high – this should thicken the sauce nicely.
Stir in the chopped parsley and serve over, noodles, rice or potatoes.