Fill a medium pot with water and bring it to a boil over medium-high heat.
Add the bruised lemongrass stalks, thinly sliced galangal, and torn Kaffir lime leaves to the boiling water; simmer for 3–5 minutes until aromatic.
Add the halved straw mushrooms and cook until almost tender, about 2–3 minutes.
Add the prawns and cook just until they turn pink and opaque, about 2–3 minutes; do not overcook.
Stir in the Thai chili paste (Nam Prik Pao) until dissolved, then gently pour in the evaporated milk or fresh milk if using; lower the heat to avoid a rolling boil.
Turn off the heat and season with fish sauce, freshly squeezed lime juice from the limes, and a small amount of sugar; taste and adjust salt, fish sauce, or lime to balance spicy, sour, and salty flavors.
Lightly crush the bird's eye chilies and add them along with the sliced or whole red Thai chilies to the soup for extra heat.
Ladle the soup into bowls and garnish with chopped sawtooth coriander (cilantro) and extra torn Kaffir lime leaves if desired.
