Bring a saucepan of water to a rolling boil for medium heat for about 2-3 minutes.
Transfer the mee hoon to a heat-resistant bowl. Pour the boiling water into the mee hoon. Cover the bowl and let it soak for 1 minute. Drain the water and toss the mee hoon to drain excess water. Put in a serving bowl and set it aside.
In a covered wok over medium heat, boil 700ml of water, lemongrass, kaffir lime leaves, galangal, shallots, and half of the salt for 2-3 minutes, or until aromatic.
Marinate the squid with half of the salt evenly. Repeat with the prawns. Set them aside.
Add 2 tbsp of tom yum paste to the herb gravy and stir until fully dissolved. Continue to boil for 30 seconds.
Wash the Shimeiji mushrooms. Stir in the Shimeiji mushrooms, Chinese cabbage stems, squid, sugar and half of the fish sauce. Continue to cook for 1.5 minutes. Then, stir in the tomatoes and chillies. Cook the Tom Yum gravy for another 1.5 minutes, or until the vegetables are cooked.
Add the lala clams and prawns. Stir them into the gravy. Let them cook for 1.5 minutes, or until the clams have opened and the prawns have changed colour.
Taste and adjust as preferred. Switch off the heat.
Stir to submerge the Chinese cabbage leaves, spring onions and cilantro into the Tom yum gravy.
Pour the lime juice and half of the Tom yum paste oil. Stir until well-combined.
Taste and adjust as preferred.
Scoop and pour the Tom yum gravy into the serving bowl. Arrange the vegetables, prawns and lala clams and squid on the mee hoon.
Drizzle 1 tbsp of evaporated milk. Garnish with chopped spring onions, cilantro leaves and the rest of the Tom yum paste oil.
Serve immediately.