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.
This is my favorite soup growing up. Porcini mushrooms, prized in Russian cooking for their immense flavor, take center stage here. I've taken my family recipe and swapped out some onions for shallot's more mellow taste. I added some thyme to highlight the mushroom flavor even further, and finally, to give this already complex-tasting soup even more depth, I cooked some sherry into the mushrooms, letting the alcohol evaporate before adding other soup ingredients. It was my best version yet, one that I will share with my family and friends and be making for years to come. After some tweaks, my favorite soup is even better than before.