Combine barley and 1 1/2 cups broth in a small saucepan. Bring to a boil over high heat; cover, reduce heat to low and simmer until the barley is tender, 30 to 35 minutes.
Meanwhile, combine porcinis and boiling water in a medium bowl and let soak until the mushrooms are soft, about 20 minutes. Line a sieve with paper towels, set it over a bowl and pour mushrooms and soaking liquid through it. 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 have released their juices and started 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, scraping up any browned bits with a wooden spoon, until most of the sherry has evaporated, about 1 minute. dd 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 celery is tender and the soup has thickened, 18 to 22 minutes.dd the cooked barley and continue cooking, stirring occasionally, until heated through, about 5 minutes. Stir in sour cream until incorporated. Garnish with chives.