Place the onions in a 3-litre (3 US quart) stockpot over a medium heat and add the salt and oil. Give it a good stir and fry to soften the onions for about 3 minutes. Then add the remaining vegetables along with the garlic and ginger paste and stir them in to coat with the oil too.

Add the ground spices and then pour in just enough water to cover. You will simmer this for some time, so do not fill the pan to the rim. The water level should be about 5cm (2in) from the top.

Bring to a simmer and then reduce the heat to low and simmer gently, covered, for about 45 minutes.

After 45 minutes, your vegetables will be much softer and the liquid will have reduced somewhat. You probably won't need to add any liquid at this stage but keep an eye on it and top it up if you need too.

Simmer for another 30 minutes. When the oil rises to the top and your veggies are good and soft, you’re ready to blend.

Using a hand-held blender, blend for about 4 minutes until the sauce is super-smooth, with no chunks and not at all grainy. This step can be done in batches in a blender. If you have a good blender, you might not need to blend as long to achieve that smooth consistency.

At this stage, the blended sauce will be quite thick (see note). Add water until the sauce is about the same consistency as full-fat (whole) milk or stock. Sometimes I need to pour the sauce into a larger bowl to do this.

Use immediately or store in the fridge for up to 3 days, or freeze in small portions of between 500ml (2 cups) and 750ml (3¼ cups) for use later. The sauce can be frozen for up to three months.