First Step: Prepare Ingredients Begin by peeling the prawns, reserving the heads and shells to create a rich seafood stock. If whole prawns are unavailable, opt for peeled prawns along with 1.5 teaspoons of shrimp paste to enhance umami. Peel and smash the lemongrass stalks to release their citrus oils, slice the galangal thinly, tear the kaffir lime leaves, and bash the garlic cloves and bird’s eye chilies to help infuse the broth. Slice the onion, wedge the roma tomato, and clean the oyster mushrooms by trimming stems.

Second Step: Build the Broth In a large pot, sauté the smashed garlic, chilies, and lemongrass briefly with prawn shells and heads, if using, to develop aroma (optional step for deeper flavor). Pour in 3 cups of water and 1/2 cup low-sodium chicken stock. Add the torn kaffir lime leaves and sliced galangal. Bring the mixture to a gentle simmer, cover, and let it cook uncovered for about 10 minutes to extract maximum herb and seafood flavors.

Third Step: Strain & Continue Cooking Strain the aromatic broth through a fine sieve to remove shells and herb remnants, returning only the clear, flavorful liquid to the pot. Set heat to low.

Fourth Step: Add Vegetables Into the clear broth, add the sliced onion and oyster mushrooms. Let simmer gently for 3 minutes to soften and absorb flavor. Next, add the roma tomato wedges and simmer for an additional minute.

Fifth Step: Cook the Prawns Add the peeled prawns into the simmering broth. Cook for about 2 minutes or until they just turn pink. Avoid overcooking to keep the shellfish tender and juicy. Remove immediately if you prefer to maintain prawn texture and presentation.

Sixth Step: Season the Soup Stir in 1 teaspoon sugar and 3 tablespoons fish sauce. Let the broth simmer for 1 minute to blend the flavors. Then, add 3 tablespoons fresh lime juice to introduce the characteristic sour note, adjusting amounts based on your taste preference. The balance of hot, sour, sweet, and salty is key to authentic Tom Yum Soup.

Seventh Step: For Creamy Tom Yum Goong Nam Khon If making the creamy version, incorporate 1 1/2 tablespoons Thai roasted chili paste and 1/3 cup evaporated milk at this stage. Stir gently until combined, warming through without boiling aggressively, to preserve the delicate creaminess while marrying the bold flavors.

Final Step: Garnish and Serve Garnish the soup with fresh coriander leaves for an herbal brightness. Serve immediately while hot with steamed jasmine rice on the side for a traditional Thai dining experience.