Heat a medium-sized pot over medium-high heat. Once hot, add the coconut oil. Once the coconut oil has melted, add the shallots to the pot and sauté for 1 minute.
Next, add the garlic and ginger to the pot and cook for 30 seconds. Then add the curry leaves and black mustard seeds cook for 1-2 minutes to release all their flavour. Stir often to avoid burning.
Next, add the curry powder, coriander, black pepper and salt to the pot and mix to combine with the other ingredients. Reduce the heat to medium and cook the spices, stirring often, for 3-4 minutes to allow them to cook through and become fragrant. If the pan is becoming too dry, add a splash of water to help the spices cook.
Once the spices are cooked, add the tomato to the pot along with the tamarind paste (if using), soy sauce, fish sauce, sugar, the Thai Birds Eye chili pepper and cook for 2 minutes, stirring often.
Add the water to the pot and bring the contents of the pot to a simmer over medium heat. Cover the pot with a lid and simmer for 15 minutes, stirring occasionally. After 15 minutes, remove the lid of the pot and add the coconut milk. Continue to simmer, uncovered, for 5 minutes to allow the flavours to combine.