In a large microwave-safe bowl, combine portabella mushrooms, oyster mushrooms and soy sauce. Cover and microwave on high for 5 minutes. Let stand 5 minutes. Strain liquid into a large stockpot, leaving mushrooms in bowl. Add the water, carrots, celery, onion, bay leaves and thyme to stock pot. Bring to a boil over medium-high heat. Reduce heat; simmer, uncovered, 30 minutes. Strain and discard vegetables from stockpot. Return liquid to pot. Stir in lemon juice, salt, pepper and cayenne pepper.
In another large microwave-safe bowl, heat 2 cups vegetable broth almost to a boil. Add dried mushrooms and let stand until softened, 2-3 minutes. Strain liquid into stockpot, reserving 1/2 cup. Combine reconstituted mushrooms with portabella and oyster mushrooms. Spoon half the mushrooms into stockpot. Spoon the remaining mushrooms into a blender. Add tofu and reserved broth. Cover and blend until smooth; stir into stockpot. Cook and stir until heated through. Sprinkle with green onions.
