Hot and sour egg drop shiitake soup

This soup from Martha Stewart is one of my favorite soups. 

http://www.marthastewart.com/341775/hot-and-sour-soup

INGREDIENTS 

• 2 cans (14.5 ounces each) reduced-sodium chicken broth 
• 2 tablespoons soy sauce 
• 1/4 to 1/2 teaspoon crushed red pepper 
• 8 ounces fresh shiitake mushrooms (about 4 cups), stemmed, caps thinly sliced 
• 3 to 4 tablespoons rice vinegar 
• 2 tablespoons cornstarch 
• 1 large egg, lightly beaten 
• 1/2 package (7 ounces) soft or firm tofu, cut into 1/4-inch cubes, drained 
• 2 tablespoons finely grated fresh ginger 
• 3 scallions, thinly sliced 

DIRECTIONS 

1. In a large (5-quart) pot, combine broth, soy sauce, crushed red pepper, and 2 cups water. Bring to a boil over medium heat. Add mushrooms; reduce heat, and simmer until tender, about 10 minutes. 

2. In a small bowl, whisk together 3 tablespoons vinegar and cornstarch. Add to pot; simmer, stirring, until soup is thickened, about 1 minute. 

3. Add egg through a slotted spoon, and stir to form ribbons. Stir in tofu. Remove from heat; let stand, covered, 1 minute. Put ginger in a small sieve, and squeeze to release juice into soup (discard solids). Taste; add remaining tablespoon vinegar, if desired. Serve sprinkled with scallions.