Many Thai restaurants in the U.S. make Tom Yum Goong with chicken stock, but in Thailand this soup is made strictly with water, which I think creates a much brighter and cleaner flavor to complement the shrimp.
Bring water, kaffir lime leaves (if using), galangal, and lemongrass to a boil over medium heat.
Add the mushrooms, fish sauce, and lime juice. Cook slowly and uncovered for 5 minutes. Add the chili and tomatoes and cook for 5 more minutes. Remove from heat. Add the shrimp and let stand for 4–5 minutes until shrimp is cooked through, stirring gently. Stir in th