- Rinse the dried mushrooms. Place them in a bowl and cover with hot water, allowing them to rehydrate for an hour. Once softened, remove the mushrooms from the water and slice them. Reserve 1 cup of the mushroom-soaking water. 
- In a small bowl, combine vinegar and white pepper, then set it aside. 
- In a large pot, add the vegetable broth, ginger, and green onion. Bring it to a boil over high heat, then reduce the heat to medium. Stir in the reserved mushroom-soaking water. Reduce the heat to low and add the rehydrated mushrooms, soy sauce, and tofu. 
- In a separate small bowl, mix the starch with 4 tablespoons of water to create a slurry. Slowly pour the slurry into the soup while stirring to thicken it. Season with soy sauce and salt, then simmer on low heat for 10-15 minutes. 
- If desired, slowly pour beaten eggs into the soup while stirring. 
- Remove the pot from heat and stir in the vinegar and pepper mixture. 
- Serve the hot and sour soup garnished with fresh cilantro sprigs and a drizzle of sesame oil. Enjoy!