To cook the soup, heat a heavy-bottomed pot over medium heat. Once hot, add the oil, followed by the shallots and celery. Sauté for about 3 to 4 minutes, or until softened and translucent.
Next, add the white mushrooms, salt and pepper. Immediately turn up the heat to high. Cook, stirring occasionally, until the mushrooms release their juices and start to brown slightly. This should take about 10 minutes or so.
Meanwhile, check on the barley. Turn off the heat once tender. Keep covered and set aside to rest.
Once the dried mushrooms are soft and rehydrated, remove them from the liquid and strain through a sieve lined with a coffee filter or cheesecloth. This will help to remove any grit. Make sure to reserve the mushroom liquid, as it will add even more flavor to the soup. Finely chop the rehydrated mushrooms and set aside.
Once the mushrooms in the pot begin to brown, don’t stir too often. To help them brown nicely, you want them be in direct contact with the surface of the pot.
Once the mushrooms are golden brown, turn the heat down to medium-high, and add the rehydrated mushrooms and garlic. Let this cook for about 30 seconds before deglazing with the sherry. Once almost all of the sherry has evaporated, sprinkle the mixture with flour. Stir in the flour and let cook for about a minute.
Next, add the reserved mushroom liquid and while stirring, add the stock. Bring the soup to a gentle boil, before reducing the heat. let the soup simmer for about 10 to 15 minutes.