In a medium pot over high heat, bring the water to a rolling boil. When the water is boiling, reduce the heat to medium and add the lemongrass, galangal, garlic, shallots, and lime leaves. Boil for 5 minutes. Stir in the tom yum paste and boil for another 5 minutes.Without stirring, add the shrimp, oyster mushrooms, and tomatoes. Increase the heat to high and cook, without stirring, for 2 to 3 minutes or until the shrimp are pink and opaque. Make sure to wait until the shrimp have had a chance to cook before stirring.Stir in the tamarind paste and cane sugar and cook for another minute. Stir in the fish sauce and lime juice. Remove from the heat. Stir in the cilantro, green onions, sawtooth coriander, and chilies. Ladle into bowls and serve.For the tamarind paste:In a small saucepan over high heat, bring the water to a rolling boil. Break the tamarind into small pieces and drop them into the boiling water. Boil for 3 to 4 minutes. The heat breaks down the tamarind pulp and makes it easier to separate the fibres. If using dried tamarind, it will start to expand. If using a tamarind block, it will absorb the water and start to soften.Strain through a fine-mesh sieve placed over a large bowl, pressing out every bit of liquid and paste. I recommend using a large bowl to help minimize the mess. Make sure to scrape the paste off the bottom of the sieve and mix it into the liquid. Discard the pulp in the sieve. Allow to cool before using. Store in an airtight container in the fridge for up to 1 week or in the freezer for up to 3 months.For the tom yum paste:In a small food processor, pulse the dried shrimp until they look stringy, almost like floss. Set aside.In a small saucepan over medium heat, heat 1 tablespoon of the sunflower oil. Add the chilies and cook until the chilies turn dark red and the skin starts to plump up and looks smooth, 1 to 2 minutes. Transfer the chilies to a stone mortar and pestle. (Alternatively, you can use a small food processor.)Return the pan to medium heat. Add the remaining ¼ cup sunflower oil and heat for 1 minute. Add the garlic and shallots and cook, stirring frequently, until cooked but not crispy, 2 to 3 minutes. The shallots will start to look transparent. Remove from the heat. Using a slotted spoon, transfer the garlic and shallots to the mortar with the chilies. Leave the oil in the pan.Pound the garlic and shallots to a fine paste. Add the paprika and pound to mix.Return the pan to medium heat and allow the sunflower oil to heat up for 1 minute. Scoop the paste out of the mortar, add to the oil, and cook, stirring frequently, for about 2 minutes. Add the shrimp floss, tamarind paste, and coconut sugar. Reduce the heat to medium-low and cook until the sugar is fully dissolved for 1 to 2 minutes. Store in an airtight container in the fridge for up to 3 days.