A lovely sour vegetarian red curry with classic Thai flavours. You can buy kaffir lime leaves in the supermarket, sometimes labeled "Australian lime leaves."
In a saucepan, simmer curry paste in 1/2 cup of the coconut milk until fragrant. Add sugar, salt, soy sauce and tamarind water. Add remaining coconut milk and bring to the boil.
Add mushrooms, tofu, watercress, lime leaves and juice. Simmer until mushrooms are cooked, about 5 minutes. Check seasoning and serve.