1. Cut each stalk of lemongrass in half crosswise. Bruise or crush the stalks with a meat mallet or a small, heavy skillet.
2. Combine lemongrass, next 3 ingredients, and, if desired, kaffir lime leaves in a 2-quart saucepan over high heat. Bring to a boil, reduce heat to medium-low, and simmer for 5 minutes. Remove pan from heat, and let steep for 15 minutes. Remove and discard solids.
3. Return seasoned stock to stove over medium heat. Add salmon, mushrooms, and tomato; simmer gently 3 to 4 minutes or until salmon is opaque.
4. Remove from heat, and stir in remaining ingredients. (If soup is too spicy, stir in another pinch of sugar.)