Bring water to a gentle boil in a large pot to create the soup base.

Juice the limes and set the juice aside so it is ready when needed.

Remove the outer layers of the lemongrass, lightly bruise the stalks, and slice them into pieces.

Slice the galangal and tear the kaffir lime leaves to release their aroma.

Prepare the chilies by removing stems and lightly crushing them for controlled heat.

Add lemongrass, galangal, kaffir lime leaves, garlic, and chilies to the boiling water and let them simmer to infuse the broth.

Lower the heat and add the shrimp, allowing them to cook gently.

Add the oyster mushrooms so they absorb the aromatic broth.

Add tomato and onion wedges and let the soup simmer briefly until tender.

Skim off any foam that appears for a clean tasting broth.

Season with fish sauce and sugar, stirring gently to dissolve.

Turn off the heat and add lime juice gradually, tasting until the balance is bright and savory.

Stir in fresh cilantro just before serving for maximum freshness.

For the creamy version

Remove half of the clear soup and set it aside.

Place the remaining soup over low heat and stir in chili paste.

Add evaporated milk and mix gently until warmed through.

Taste and adjust seasoning with lime juice or fish sauce as needed.