Add raw stew beef to slow cooker and season generously with salt and pepper. Add stock, celery, onion and garlic. Stir and cook on high for 4 hours.
Dice potatoes (cutting them up just before you add them will keep them from discoloring). Add potatoes, carrots, mushrooms and pearl barley and cook on high for 2 more hours. Season to taste with salt and pepper and serve.