Bring barley and 1 1/2 cups broth to a boil in a small saucepan over high heat. Cover, reduce heat to low and simmer until tender, 30 to 35 minutes.

Meanwhile, combine porcinis and boiling water in a medium bowl and soak until softened, about 20 minutes. Line a sieve with paper towels, set it over a bowl and pour in mushrooms and soaking liquid. Reserve the soaking liquid. Transfer the mushrooms to a cutting board and finely chop.

Heat butter and oil in a Dutch oven over medium-high heat. Add shallots and cook, stirring often, until softened, about 2 minutes. Add white mushrooms and cook, stirring often, until they start to brown, 8 to 10 minutes. Add the porcinis, celery, sage, salt and pepper and cook, stirring often, until beginning to soften, about 3 minutes. Sprinkle flour over the vegetables and cook, stirring, until the flour is incorporated, about 1 minute. Add sherry and cook, stirring, until most of the sherry has evaporated, about 1 minute.

Add the soaking liquid and the remaining 3 cups broth; increase heat to high and bring to a boil. Reduce heat and simmer, stirring occasionally, until the soup has thickened, 18 to 22 minutes.

Add the cooked barley and continue cooking, stirring occasionally, until heated through, about 5 minutes more. Stir in sour cream until incorporated. Garnish with chives.

Shopping tip: Look for mushroom broth in aseptic containers in well-stocked supermarkets or natural-foods stores.

Ingredient note: Sherry is a type of fortified wine originally from southern Spain. Don't use the "cooking sherry" sold in many supermarkets--it can be surprisingly high in sodium. Instead, purchase dry sherry that's sold with other fortified wines in your wine or liquor store.

MAKE AHEAD TIP: Prepare soup through Step 4. Cover and refrigerate the soup and cooked barley separately for up to 3 days. To serve, combine (Step 5) and reheat.