Tip the prepared chillies, onions and garlic into a food processor and blend until quite smooth – the seeds will still be visible.

Put the oil in a large saucepan over a moderate heat, then add the chilli, onion and garlic paste. Fry for 8-10 mins, stirring frequently, to soften the ingredients, but take care that they don't burn.

Add the sugar, tomatoes and vinegar, and simmer for a further 15 mins until the sauce has thickened a little. Blend again, if desired, taste and adjust the seasoning if needed. If you like a very smooth texture, strain through a fine sieve. Leave to cool, then store in a sterilised jar. Will keep chilled for up to a month in an airtight jar.