Prepare all the ingredients.
Place a stockpot on the stove and set the heat to medium. Pour the stock into the pot, and add the herbs: lemongrass, galangal, cilantro roots, shallots, kaffir lime leaves, bird's eye chillies, and 100g tomato wedges. Give the soup a gentle stir. Cover the pot and let it come to a rolling boil, for about 4 minutes.
While waiting, marinate the seafood. Marinate the squid flowers with 1/4 tsp of salt, rubbing to distribute it evenly. The clams do not need to be marinated, but they should be washed thoroughly.
Remove the horns, whiskers and legs from the prawns. Devein them by piercing the back of the prawns perpendicularly with a toothpick, in between the second and third last shell. Pull the vein out gently and discard it. Then, cut the back of the prawn to make it easy to remove the shells once cooked. Repeat for all the prawns. Then, marinate them with 1/4 tsp of salt on each side. Set the seafood aside.
When the soup is boiling vigorously, add the Tom Yum Paste, stirring until it is well combined. Then, let it come to a rolling boil.
Once the Tom Yum soup is boiling vigorously again, wash the mushrooms.
Add the squid and clams. Gently add each of the prawns to the surface of the soup, followed by oyster mushrooms and white shimeji mushrooms. Do not stir the pot once the seafood has been added to prevent the soup from becoming fishy. Cover the pot and let it boil for 2 minutes.
Open the cover and rotate the mushrooms so they are submerged in the soup. As all the seafood will be cooked by now, give the soup a taste. This helps us to adjust the seasonings as preferred.
Add the seasonings: 1/2 tap of salt, sugar and fish sauce. Stir until they are well combined.
Add the tomato wedges, parsley, and spring onions. Switch off the heat. Stir the soup and submerge the vegetables.
Taste and adjust as preferred.
Add lime juice, and stir gently to combine. Garnish with deep-fried chillies. Dish it out to individual bowls. Just before serving, garnish with parsley and spring onions and pour the evaporated milk all around the bowl.