Gently heat the oil in a large pan.
Add the onions and salt. Fry on a gentle heat for 10 minutes.
Add the garlic and ginger to the pan and fry for another 5 minutes.
Add the chopped pepper and carrots and give a quick stir.
Add the curry masala/powder and fry gently while stirring for 2 minutes. If the spices are sticking add a splash of water to loosen them.
Add the tomato puree and stir in for another 30 seconds
Add the water and give a good stir to make sure it's not sticking at the bottom of the pan. Bring up to the boil.
Once the sauce has come to the boil turn the heat down so it is just simmering. Add the sugar and the coriander stalks.
Continue to cook for around 45 minutes.
After 45 minutes check that the carrots are soft. Once they are, the sauce can be blended using an immersion blender or stick blender. You could use a food processor or jug blender but you would have to do it in batches and let it cool first.
Once the sauce is blended you can continue to cook it for another 30 minutes to 1 hour. If you are going to be freezing some of the sauce you may want to continue cooking so the sauce is reduced and thicker. It will take up less space in the freezer and you can just thin it down when you want to use it.