Prepare the aromatics: remove the tough outer layers of the lemongrass 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. Tear the 6 whole kaffir lime leaves 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, 6 slices of galangal, 6 whole kaffir lime leaves, 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 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 and 2 whole tomatoes wedges (if using). Simmer for 3 minutes until the 8 oz mushroom are just tender.

Add the 1 lb shrimp and 3 whole green onions. Cook for 2–3 minutes, just until the 1 lb shrimp are pink and curled. Do not overcook — 1 lb shrimp go from perfect to rubbery quickly.

Remove the pot from the heat. Stir in the fresh 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 cilantro. Serve with 1 whole lime wedges on the side.