In a wok, heat the oil and fry the onion until soft, adding the sugar. Add chopped tomatoes and stir. Add garlic, purée, salt, butter and mixed herbs. Simmer on low heat until salsa is at desired thickness. Chill in the fridge and enjoy with any snack or main meal.
You can make this salsa more spicy by adding peppers, chilli powder and fresh chillies.