Sprinkle curry powder, salt and turmeric over the chicken and set aside. In a blender or food processor mix onions, garlic, paprika, chilli powder and ginger into a thick paste.
Heat the oil in a deep pan and fry the paste over a medium heat for about 10 minutes. Add the chicken and turn up the heat stirring for another few minutes. Add the tomatoes and stir for a further 5 minutes. Add 1 1/2 cups of warm water (enough to cover the chicken), the lemongrass and fish sauce. Stir well and bring to a simmer.
Turn down the heat to low, partially cover the pan and very gently simmer for at least 30 minutes until the chicken is tender.
Remove the lid and simmer for a further 10-15 minutes to reduce the sauce. Fish out the lemongrass - check for seasoning and serve!
I like to serve mine with broken rice as I love how it soaks up the sauce. For those who don't already know broken rice just Jasmine rice - broken in half! You can also use regular jasmine rice obviously.
I often add some vegetables towards the end of cooking. My favourite is to add a cup of green beans - helps with your 5-a-day!You can also think about adding things like tomatoes, potatoes, cauliflower and carrot too.