How to Make It
In a large soup pot, bring the stock, mushroom stems, ginger and garlic to a simmer. Cook over low heat for 30 minutes. Strain the stock into a large bowl and return it to the pot. Season the broth with salt.
Bring the broth to a boil; add the chicken. Cook until the chicken is white throughout, about 4 minutes. Divide the mushroom caps, tofu and scallions among eight soup bowls and serve, passing the broth and sesame oil at the table.