Slice the onion into thick slices, leave the skin on. Heat the BBQ or broiler to high. Roast off the vegetables, turning or flipping to get a consistent char. (I like to put the tomatoes and tomatillos on a tray so the juices will be kept as well.)
Put the peppers in a bowl, cover with saran wrap and set aside.
Remove the outer ring of the onions and give a rough chop and place in the food processor.
Remove the skins and seeds from the peppers, and squeeze the garlic out of the skin, and place in food processor.
Remove the skins from the tomatoes, placing them and all the remaining ingredients into the food processor.
Blend until well mixed, then proceed to add more salt or lime juice according to taste.