place the tomatoes and chillies in a pan with enough water to cover. Bring to the boil and poach gently for 7-10 minutes
Finely chop the onion, garlic and coriander
remove the tomatoes and chillies from the pan and keep back the simerring liquid.
Skin the the tomatoes and place them with the chillies into blender or food processor. Blend briefly, then add the remaining ingredients and blend some more to achieve a coarse or smooth salsa depending on yor taste. You can thin your salsa with a little of the simmering liquid. Finally season to taste
Alternatively use a molcajete to create a chunkier texture.