Bring 2 liters of water to a boil in a large pot or saucepan.

Squeeze 10-15 limes to yield 8-12 tablespoons of lime juice; set aside for later use.

Prepare the 4 stalks of lemongrass by tearing off the outermost leaf and discarding it. Lightly pound the stalks with a mallet to release their flavors, then slice them diagonally into 1-inch strips.

Chop the 1-inch chunk of galangal into slices.

Coarsely tear the 10 kaffir lime leaves to help release their aromatic oils.

Peel the 5 cloves of garlic.

Remove the stems from the 10 Thai chilies. You can either slice them in half or lightly pound them on your cutting board, being careful of flying seeds. For less heat, you can remove the seeds.

Add the prepared lemongrass, galangal, kaffir lime leaves, garlic, and Thai chilies to the boiling water. Cover the pot to help it return to a boil and infuse the herb flavors quicker.

Prepare the ½ kilo shrimp by cleaning them and chopping off tentacles if using freshwater shrimp.

Boil the soup with the herbs for about 10 minutes to allow the flavors to develop. Then, add the shrimp to the pot and reduce the heat to low.

Rinse the 300 grams of oyster mushrooms and add them to the soup.

Slice the 2 roma tomatoes and 2 medium white onions into large, chunky wedges. Add them to the tom yum.

Skim off any foam that builds up on the surface of the soup, particularly from the shrimp, and discard it.

Boil the tom yum gently for about 2-3 minutes, ensuring the mushrooms and onions soften.

Stir in 6 tablespoons of fish sauce and 2 teaspoons of sugar.

Boil for another minute, then turn off the heat to prevent overcooking the ingredients.

Add 8 tablespoons of the reserved lime juice to the soup.

Taste the tom yum and adjust seasoning as needed, adding more fish sauce for saltiness, sugar for sweetness, or lime juice for sourness, until the flavor is perfectly balanced.

Chop a handful of fresh cilantro and stir it into the soup. Adding the lime juice and cilantro off the heat helps preserve their fresh and vibrant flavors.

Separate approximately half of the clear tom yum soup into another pot if you wish to enjoy both versions.

Turn on the heat to low for the pot containing the soup designated for the creamy version.

Add 3 heaping tablespoons of Thai roasted chili sauce (nam prik pao) to the soup and stir until it is thoroughly combined.

Pour in 10 tablespoons of evaporated milk and mix well.

Heat the creamy soup for about 1 minute, allowing it to warm through without boiling vigorously.

Taste the creamy tom yum and adjust the seasoning. The roasted chili sauce and evaporated milk will alter the flavor profile, so you may need to add additional lime juice or fish sauce to achieve your desired balance of sourness and saltiness.