Step 1Place the mushrooms in a bowl and pour the water over them. Let rehydrate, 30 minutes. (Put a plate on top to keep them submerged.) Lift the mushrooms from the bowl with a slotted spoon. Pour the liquid into a sieve lined with cheesecloth. Reserve the liquid and set aside. Mince 2 mushrooms and set aside for garnish. Reserve the rest.
Step 2Heat the butter in a 4-quart saucepan over medium-low heat and add the onion. Cook, stirring occasionally, until softened, about 5 minutes. Add the garlic and cook 1 minute. Stir in the rice. Stir in the reserved mushroom liquid, then the chicken broth. Add the rehydrated mushrooms. Bring to a simmer. Cover and cook 30 minutes.
Step 3Remove one-half cup of the mushrooms with a slotted spoon. Roughly chop and set aside. Puree the soup in batches, then return to the saucepan and stir in the cream. Add the salt. Stir in the roughly chopped mushrooms. Ladle into bowls. Garnish each bowl with a teaspoon of the sour cream. Sprinkle with cracked pepper, the mushrooms for garnish and the chives.