Boil water in a medium pot.
Add lemongrass, galangal, shrimp chili paste and onion. Cook until fragrant.
Add tomatoes, shrimp and mushrooms.
Once the shrimp is cooked, season with fish sauce, salt and sugar.
Stir thoroughly and add the lime juice, green onion, cilantro, Thai parsley and lime leaves before serving. (If not spicy enough add 1 to 5 crushed chilies)