Heat the oil in a saucepan or frying pan. Add the onion and cook over a medium heat for 3 to 4 minutes, until soft.
This sauce makes a great cover for some hidden veg to help towards your 5 A Day. You could try adding finely chopped peppers or courgettes to cook with the onions. And if the sauce is a bit chunky, simply blend or purée it at the end.