How to Make It
In a large saucepan, combine the stock with the smashed ginger and bring to a boil. Cover and simmer over low heat for 15 minutes. Strain out the smashed ginger pieces.
Meanwhile, heat the oil in a large nonstick skillet until shimmering. Add the shiitakes and cook over moderately high heat, stirring occasionally, until tender and just beginning to brown, about 8 minutes. Add the minced ginger and garlic and cook until fragrant, about 1 minute. Season with salt and pepper. Stir the shiitakes and watercress into the soup. Ladle into shallow bowls and serve.