In a bowl, combine dried mushrooms with hot water, let stand 30 minutes. Strain through a sieve lined with paper towel, reserving liquid. Remove stems. Pat dry and chop finely. Set mushrooms and liquid aside, separately.
In a skillet, heat oil over medium heat for 30 seconds. Add beef, celery, onion and reserved mushrooms; cook, stirring for 1 minute. Reduce heat to low, cover and cook until vegetables are softened, about 8 minutes. Add garlic, ginger, pepper and five spice; cook, stirring for 1 minute. Add fresh mushrooms and barley; toss to coat. Add stock and reserved mushroom liquid; bring to a boil over high heat. Transfer to slow cooker stoneware.
Cover and cook on low for 8 hours or on high for 4 hours, until barley is tender. Stir in soy sauce.