In a small saucepan, bring 1 cup stock to boil. Remove from heat and add porcini mushrooms. Let stand at least 10 minutes or until softened. Drain, reserving liquid, and chop mushrooms.
Heat oil in a saucepan over medium heat. Add shallot and celery and sauté 4 minutes or until softened and golden.
Add porcini and fresh mushrooms and sauté 5 minutes. Add remaining stock and soaking liquid and bring to boil. Reduce heat and simmer, partially covered, 10 minutes or until mushrooms are soft. Stir in chives, salt and pepper, and simmer 3 minutes. Garnish each portion of mushroom and chive soup with sour cream.