In a large pot, bring the chicken or shrimp stock to a boil over medium-high heat.

Add the prepared lemongrass, galangal, and torn kaffir lime leaves to the boiling stock.

Reduce the heat to medium and let the aromatics simmer for about 5 minutes until the broth is fragrant.

Stir in the Thai roasted chili paste (Nam Prik Pao), fish sauce, sugar, and crushed fresh chilies until the paste dissolves completely.

Add the mushrooms and cook for 2 to 3 minutes until they start to soften.

Add the prawns to the pot and cook for about 2 minutes, or just until they turn pink and opaque.

Pour in the evaporated milk and stir gently to combine, allowing the soup to warm through without boiling vigorously.

Turn off the heat immediately and stir in the fresh lime juice; adding it off the heat preserves its bright, zesty flavor.

Taste the soup and adjust seasoning with more fish sauce or lime juice if needed to achieve the perfect hot and sour balance.

Ladle the soup into bowls, garnish generously with fresh cilantro, and serve immediately.