Place the onions in a 3-litre (3 US quart) stockpot over medium heat and add the salt and oil. Give it a goodstir and then add the remaining vegetable and just enough water to cover.

You will be simmering this for some time so do not fill the pan to the rim. The water level should be about 5cm (2 inches) 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 some. Add the remaining ingredients and top up with water so that the water level is again about 5cm (2 inches) from the top. Take this as aguide. If you don’t need to add water at this time, you don’t need to.

Continue simmering for another 30 minutes. When the oil rises to the top and your veggies good and soft, you’re ready to blend. Carefully skim the seasoned oil off the top for use in your curries or just leave it in the sauceif you like.

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 blendas long to achieve that smooth consistency.

At this stage, the blended sauce might be quite thick. Add water until the sauce is about the same consistency as full fat milk or single cream. 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) or 750ml (3 cups) for use later. The sauce can be frozen for up to three months.