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.
Vegetable oil for cooking the onions. Flour to thicken the sauce.