A great cold weather soup that really is thick and satisfying. My parents use to have a deli restaurant on Connecticut Ave in D.C. and my mother use to make big batches of this soup to sell at lunch time. I make this all the time, myself. You can use a variety of mushrooms; cremini, button, brown, re hydrated dried mushrooms. I've made it with white button and cremini or a mixture. I like thick soup, and I'll use more than 1/2 cup barley.
In a large heavy pot over medium heat, melt the butter. Add the onion, carrots, parsnip and salt and cook, stirring frequently, until the onion is softened, about 10 minutes.
Add the mushrooms and continue to cook, stirring only occasionally, until all the mushrooms are tender, about 15 minutes more, adding more butter if necessary.
Season with pepper and thyme and stir to combine. Add the stock, sherry if desired, and barley, increase the heat to medium-high and bring almost to a boil.
Reduce the heat to medium-low, cover partially and simmer gently until the barley is completely cooked and tender, about 1 hour. Check the liquid level during simmering; if a thinner consistency is desired, add additional stock.
May keep warm over low heat, stirring occasionally, for up to a couple of hours, or you may cover and refrigerate overnight and rewarm over low heat. To serve, garnish with parsley.