Place dried mushrooms in a medium bowl and cover with boiling water. Let steep to soften, about 15-20 minutes before straining, reserving 1 1/2 cups of the mushroom water. Roughly chop softened mushrooms and set aside.
Heat olive oil in a large soup pot then add diced onions. Saute to begin softening, along with a pinch salt and pepper, for about 2-3 minutes before stirring in the diced carrots and celery. Season to taste again with another pinch salt and pepper.
Stir in the reserved, chopped softened shiitake mushrooms and sauté another 2-3 minutes before adding the reserved mushroom water. Using a wooden spoon, be sure to scrape up and browned bits from the bottom of the pan before adding in the chicken broth and another pinch salt and pepper. Bring to a boil then add in the sliced white mushrooms and barley. Return to boil then reduce to low and let simmer 45 minutes or until barley is tender. Check for seasoning then adjust accordingly. Garnish with parsley to serve.