Step 1

Heat the oil in a soup pot. Add the onion and sauté over low heat until golden.

Step 2

Add the bok choy, tomatoes, mushrooms, broth, and pepper. Bring to a rapid simmer, then lower the heat. Cover and simmer gently until the mushrooms are tender-crisp, about 6 to 8 minutes.

Step 3

Add the remaining ingredients except the last three. Taste frequently as you add the vinegar and hot seasoning to your taste, until the soup tastes "hot and sour" to your liking.

Step 4

Dissolve the arrowroot or cornstarch in about 1/4 cup water. Slowly drizzle into the soup while stirring. Stir in the bok choy and scallions and serve.