Ingredients
1 tbsp flavourless oil 
1 onion, finely chopped 
10g garlic cloves, crushed or very finely chopped 
10g ginger, finely grated 
3 tbsp curry powder 
1 tsp ground turmeric 
¼ tsp ground white pepper 
2 tbsp plain flour 
400ml stock (chicken or vegetable work well) 
200ml coconut milk 
1 tbsp soy sauce 
2 tsp sugar 
1½ tsp rice vinegar 
½ tsp salt 

Method
Put the oil into a wok and heat on medium.  Add the onions and cook for 5-7 minutes until soft and translucent.  You don't want the onion to colour at all so make sure you stir and reduce the heat if need be.  Add the garlic and ginger and cook for a further 1-2 minutes. 

Add the curry powder, turmeric and white pepper and stir cooking for a further minute.  Add the flour, stirring and cooking for further minute - you want to cook out the raw taste.  Now slowly add the stock a little at a time, stirring between each addition.  This might sound weird but think about making a bechemel - the only difference being you are using stock rather than milk.  Once all of the stock has been added, you need to either blitz your sauce with a stick blender/food processor or, strain out the onion garlic and ginger - this sauce should be totally smooth.  

Once blitzed/strained, put back on to a medium heat and add the coconut milk, soy sauce, sugar and rice vinegar (if using) and stir.  Check you are happy with the seasoning, adjust with salt and sugar as necessary. Serve hot.