Sliced scallions and chopped fresh parsley to garnish
Place the dried mushrooms in a small bowl and cover them with the hot water. Let soak for 20 minutes. Remove the mushrooms, reserving the soaking liquid, and finely chop the soaked mushrooms. Return them to the liquid.
Heat the oil in a large soup pot or Dutch oven over medium heat. Add the onions, garlic and thyme, and season with salt and pepper. Sauté, stirring occasionally, until the onions are tender, about 5 minutes. Add the mushrooms and stir until the have become tender, and any liquid that they released has evaporated.
Stir in the flour and stir for one minute until it is well distributed. Add broth and increase the heat to high. When the soup is simmering add the barley to the soup. Continue to simmer, uncovered, until the barley is tender, about 45 minutes, stirring from time to time. Add more broth if the soup is too thick. Check the seasonings towards the end and add salt and pepper if needed.
Serve hot, with scallions and parsley sprinkled on each serving.