In a small bowl, add the curry paste, cumin, coriander, cinnamon, nutmeg and cardamom and stir to combine. It will become a thick, chunky paste.
Heat a large Dutch oven or heavy pot on medium high. Add the oil, along with the onions and curry paste mix and stir until the onions have softened slightly and the paste has caramelised and is sticking to the pan.
Add the mushrooms and potatoes and stir to coat in the paste.
Add the brown sugar, tamarind, salt and half the coconut milk and stir well.
Reduce heat to medium, cover and cook until the potatoes are tender, 20 to 25 minutes.
Once the potatoes are tender, add the remaining coconut milk and soy sauce and stir to combine.
Taste and season well with salt.
