Bring the pot of water to boil, then add the tamarind powder, fish sauce, chicken bouillon powder, salt, sugar, tom yum paste and kaffir lime leaves in. Mix until well combined.
Wash and slice the mini king oyster mushrooms in half, then add them to the broth and turn the heat down to a medium simmer while you work on the other ingredients.
In a small saucepan, heat up 1 tbsp oil and brown the 3/4 of the chopped garlic on low heat for 3 minutes or until golden, then pour the contents into the stock.
In a large wok, add 5 tbsp oil and add the remaining garlic in along with the ginger on medium heat. Stir for 30 seconds.
Turn the heat to high and add the fish in along with the lemongrass. Cook for 3 minutes or until the outsides of the fish are seared.
Bring the broth back to a boil and pour the pan fried fish and lemongrass in, then add the prawns and tomatoes to cook for 4 minutes or until just cooked.
Meanwhile, bring a large pot of water to a boil and cook the rice noodles for 5 minutes or until al dente. Drain and run under cold water to cool.
Serve the broth and toppings hot with the noodles and fresh garnish!