Prepare the aromatics: remove the tough outer layers of the 2 stalks of lemongrass (切成2英寸块并用刀背拍打) and cut into 2-inch pieces, then bruise each piece firmly with the flat side of a Knife. Smash the 4 cloves of garlic cloves and chilies. Slice the 6 slices of galangal (或用新鲜生姜代替；替代方案：用新鲜生姜代替). Tear the 6 whole kaffir lime leaves, 撕 in half — this releases their fragrance.

In a large Saucepan, combine the 5 cups low-sodium chicken broth and 2 cups water over medium-high heat. Add the 2 stalks of lemongrass (切成2英寸块并用刀背拍打), 6 slices of galangal (或用新鲜生姜代替；替代方案：用新鲜生姜代替), 6 whole kaffir lime leaves, 撕, smashed chilies, and 4 cloves of garlic. Bring to a boil.

Once boiling, reduce the heat to medium and let the broth simmer for 10 minutes to fully infuse all the aromatics. The broth should smell intensely fragrant.

If using, stir in the 1 tbsp thai roasted chili paste (optional) until dissolved — it adds a rich, smoky depth. Then add the 1 tsp granulated sugar and 3 tbsp fish sauce. Taste the broth; it should be savory and slightly salty at this stage (the 3 tbsp lime juice will add sourness next).

Add the 8 oz mushroom, 切片 and 2 whole tomatoes (切成楔形) wedges (if using). Simmer for 3 minutes until the 8 oz mushroom, 切片 are just tender.

Add the 1 lb shrimp, 去壳和去肠 and 3 whole green onions (切成1英寸块). Cook for 2–3 minutes, just until the 1 lb shrimp, 去壳和去肠 are pink and curled. Do not overcook — 1 lb shrimp, 去壳和去肠 go from perfect to rubbery quickly.

Remove the Pot from the heat. Stir in the fresh 3 tbsp lime juice. Taste and adjust seasoning: more 3 tbsp fish sauce for saltiness, more 3 tbsp lime juice for sourness, a pinch more 1 tsp granulated sugar to balance. The broth should be a vibrant balance of sour, salty, and spicy.

Ladle the soup into bowls. Garnish generously with fresh ½ cup cilantro, 粗碎，新鲜 (optional). Serve with 6 whole kaffir lime leaves, 撕 wedges on the side.