Put the tomatoes in hot water for about 10 mins, remove the skin and chop. Pierce the capsicum with a fork and hold over flame until the skin blackens. Remove from heat, rub off the burnt skin and chop.
Heat the oil and fry the onions for 1/2 a min.
Add the remaining ingredients and cook for 3-4 minutes.
Put salsa in the refrigerator, and serve cold with nachos.