Heat the ghee in a medium-sized casserole over a medium to low heat, and add the onions. Soften for about 20–30 minutes, being careful not to let them catch by stirring regularly.
Next, add the garlic and ginger and soften for a further 10 minutes.
Add the spices and allow to cook for a couple of minutes, then stir in the tomato purée and cook for another minute or two.
Pour in the blitzed tomatoes and bring everything to the boil, then reduce the heat and cook slowly for 40 minutes until thickened and reduced and full of flavour, making sure to give the sauce a good stir every so often.
At this stage add the cashew nuts, salt and coconut sugar. Reduce for a further 10 minutes before transferring to a food processor. You want a really smooth creamy texture, so allow the sauce to blend for at least a minute.
Check for seasoning. Now you have your sauce base!