After spending many years trying to make curries like indian resaturants, I was delighted to be introduced to Kris Dhillon's book, The Curry Secret. In March 2009, Kris Dhillon released a new book, The New Curry Secret. It's definitely worth having both books as they contain different recipes. Although this post was written sometime ago. I'm updating today 1.10.09 as I have received an email from Kris Dhillon informing me she now has a blog too, which can be seen
here

Ingredients:-
4 tbsp olive oil
1kg sliced onions
1 tsp salt
50g fresh ginger, coarsely chopped
50g garlic, coarsely chopped
1.5 litres water
1 small can (225g) tomatoes
1 tsp tomato puree
1 tsp turmeric
1 tsp paprika
Step 1:-
Heat all but 1 tablespoonful of the oil in a large saucepan with a tight fitting lid and add the sliced onions. Stir on high heat for a minute or two and then turn heat down a little. Continue to cook the onion, stirring frequently, for about 10 mins until onion is transparent and beginning to brown.
Add the salt, stir well and turn heat down to as low as possible. Cover pan and continue to cook the onion for about 20 mins, stirring three or four times during that period. The addition of salt will prevent onion from browning and help it soften and break down, but be careful it does not stick to bottom of pan.
Add ginger & garlic to onion. Turn heat up a little and cook for a minute or two, until aromatic.
Add the water and bring to the boil. Turn the heat down and simmer the mixture, partly covered, for 20-25 mins.
Leave to cool.
Step 2:-
Once cooled,ladle half the mixture into a blender jug and blend until smooth. To ensure that the sauce is really smooth, blend for about 2 mins. Pour blended onion mixture into a clean pan or bowl and repeat with the other half of the onion mixture.
Wash and dry the saucepan. Reserve about 4 tablespoons of the sauce at this point to use for cooking the meat (details in other recipes).
TIP: Freeze at this stage (see note below) if you want to keep the sauce for up to 2 months. You can freeze the finished sauce for up to 1 month.
Step 3:-
Place contents of can of tomatoes into the rinsed blender jug and blend until smooth.
Place the cleaned saucepan onto medium heat and heat the remaining oil. Add the tomato puree, turmeric and paprika and cook for about 30 seconds. Add the blended tomatoes and bring to the boil. Turn down the heat and cook, stirring occasionally, for about 10 mins.
Add the blended onion mixture to the tomatoes and bring to the boil again. Turn down the heat so that the sauce simmers gently.
As the sauce simmers, you will notice froth rises to the surface. This needs to be skimmed off. Continue to simmer, skimming as necessary, for 20-25 mins.
Use immediately or cool and refrigerate for up to 4 days or freeze for 1 month.
This is the basic sauce required for making many dishes, individual dishes require other ingredients to be added (see various recipes)
Note: Cup measurements are 150ml. If freezing the sauce, freeze in 150ml portions.
The above curry sauce makes enough for about eight main course dishes.