Tom Yum Thai soup is the most popular recipe of all the Thai soups. It usually includes shrimp, but a vegetarian version is just as satisfying. Tom Yum Soup is famous worldwide, and could possibly boost the immune system and help fight off cold and flu viruses.
Pour stock into a soup pot. If making the stock from cubes or powder, but sure to make it strong (if it tastes good on its own, it will make a better soup). Add the prepared lemongrass, plus the lime leaves, chili, garlic, and galangal or ginger. Bring to a boil and continue boiling for 5 minutes, or until broth is very fragrant.
Add the mushrooms. Reduce heat to medium and simmer for 5 to 8 minutes, or until mushrooms are soft.
Add the bok choy and cherry tomatoes. Gently simmer 1 to 2 more minutes (bok choy should remain on the crisp side).
Reduce heat to low and add the coconut milk if using, sugar, soy sauce, and lime juice. Add the soft tofu and gently stir.
Do a taste-test, adding more chili or chili sauce if not spicy enough. If not salty enough, add more soy sauce or a little more stock or salt. Add 1 more teaspoon sugar if too sour. If too salty or sweet, add another squeeze of lime juice.
To serve, ladle soup into bowls with fresh basil and coriander sprinkled over.