Pour chicken stock into a medium-sized pot or saucepan.
Add lemongrass and galangal to the chicken stock, then bring it to a boil over medium-high heat.
When the stock starts to boil, add Tom Yum Paste (see notes below).
Add kaffir lime leaves, mushrooms, tomatoes and Thai bird chili (if using) to the soup.
When the soup reaches a rolling boil, add the shrimp. Cook for 1–2 minutes, just until the shrimp turn pink and opaque.
Stir in the sugar, fish sauce, and lime juice.
Remove the lemongrass, galangal slices, and kaffir lime leaves before serving.
When the shrimp are cooked (this should take only 1 - 2 minutes), transfer the soup to a serving bowl.
Garnish with fresh cilantro leaves, then serve with lime wedges on the side. Enjoy!
Notes
* You may substitute the 4 cups of chicken stock with 4 cups of water + 1 bouillon cube* Tom Yum paste varies in strength by brand. Start with 1 tablespoon, taste, then add more if needed.* The shrimp may be peeled or whole with the shell on; your choice.