Finely chop the onion, garlic and chilli.
Heat the oil in a large saucepan and gently sauté the chopped vegetables, until soft.
Meanwhile, peel and dice the sweet potatoes and carrot. Chop the tomatoes.Add these to the pot, stir and cook for another minute then add the spices & tomato purée.
Add enough water to just cover the vegetables and add the stock powder.
Bring to the boil, cover, turn down the heat and cook for 15-20 minutes until the sauce is thick and the vegetables are tender.
Turn off the heat. Remove the bay leaf and cinnamon bark. Add your chosen herb. Season to taste, then process with a stick blender until you get a thick, smooth sauce.
Let it cool then transfer it to jars, freezer containers or bags and chill or freeze until you're ready to use.