Chip shop curry sauce is a uniquely British thing - it's a pale imitation of the rich gravies that we know and love from the Indian sub-continent. It's quick and easy to make, and delicious poured over chips.
Whizz everything into a blender until you have a smooth paste the consistency of paint. Return to the pan, and check the seasoning. If your sauce seems too runny, you can sift in some flour to thicken it.