Cut Aubergines into 1 inch cubes, peel sweet potato and cut also into 1 inch cubes.
Bruise the lemongrass and cut off the dry top half. Put lemongrass into processor with the coriander root and stems, garlic, ginger, chilli, lime leaves and 2 tbsp water. Blend to a paste.
In a large fry pan heat 3 tbsp oil until almost smoking. Fry Aubergine in small batches until golden, remove and drain on kitchen paper.
Place a large saucepan over a medium heat and add the remaining 1 tbsp oil, when hot add onions and cook, stirring from time to time for about 5 mins until translucent. Add the paste together with the roasted spice mix and cook, stirring for 3-4 mins to release the flavours.
Pour in the coconut milk, then add the tamarind water, fish sauce and sugar. Stir well, bring to the boil, than add the sweet potato.
Turn down the heat and simmer for 10 mins, then add the browned aubergine and cook for a further 5 minutes.
In a separate pan cook the spinach quickly until just wilted. Tip into a sieve and refresh under cold running water and squeeze dry.
Just before serving add the spinach to the curry and gently warm through. Taste and adjust seasonings if necessary with sugar or fish sauce. You are looking for a balance of sweet, sour, hot and salty flavours.