In a small saucepan, bring the barley and 1 1/2 cups broth to a boil. Cover, reduce heat to low, and simmer for 30-35 minutes until tender.
Place the dried porcini mushrooms in a bowl and pour over 2 cups boiling water. Let soak for 20 minutes, then strain through a paper towel-lined sieve, reserving the liquid. Chop the soaked mushrooms finely.
Heat butter and olive oil in a Dutch oven over medium-high heat. Add shallots and sauté for 2 minutes until soft, then stir in white mushrooms and cook for 8-10 minutes, stirring often, until golden brown.
Stir in the chopped porcini mushrooms, celery, sage, salt, and pepper. Cook until the celery begins to soften, about 3 minutes. Sprinkle in flour, stirring constantly for 1 minute to coat the vegetables.
Pour in the sherry, stirring until mostly evaporated. Add the reserved porcini liquid and remaining 3 cups broth, then bring to a boil. Reduce heat to simmer and cook for 18-22 minutes until the soup thickens.
Stir in the cooked barley and continue cooking for 5 minutes until heated through. Remove from heat and mix in sour cream. Garnish with minced chives and serve warm.