In a medium bowl, pour 2 cups boiling water over the mushrooms and allow to infuse 1 hour. Strain over a sieve, pressing out solids; reserve the liquid.
In a large stockpot, over medium heat, melt 2 tablespoons butter and warm the 3 tablespoons olive oil. Sweat the onion, shallots, and thyme for 10 minutes until softened and light golden. Add garlic and cook 1 minute.
Add 2 tablespoons butter and the reserved mushrooms to the pot, and cook 5 minutes. Add sherry and cook, stirring, 3 minutes, until mostly evaporated. Add the mushroom-infused liquid, chicken stock, barley, salt, and pepper. Cover, and simmer on low heat 1 hour.
Add the potatoes, raise the heat to medium, and cook 15 to 20 minutes, until the potatoes are fork-tender. Serve as is, or with a dollop of sour cream, if you like.