Preheat oven 200 C. Heat a large cast iron or heavy bottomed griddle over high heat until almost smoking. Place the chilli, tomatoes and garlic on the griddle and leave to blacken in the pan. Use tongs to move the ingredients around, but don't be tempted to fiddle with them too much. You want them to char a bit. As they blacken, place them on a baking tray and then in a hot oven for 5 minutes until the skins of the tomatoes are charred and will come off easily.
Remove the baking tray from the oven and leave to cool for a few minutes. Deseed the chilli and remove the skin which should peel away easily. Mince into a fine paste and place in a medium sized bowl. Remove the blistered skins from the garlic and mash into a paste and add to the chilli. Finally remove the skins from the tomatoes and roughly chop them. Add them along with any juices left over in the baking tray to the bowl with the chilli and garlic. Add the finely chopped onion and mix through with a fork. Add the juice of 1/2 a lime, a splash of vinegar, a pinch of sugar and some salt. Taste and add more seasoning or lime juice as required. If the sauce is too hot you can add a little more sugar to bring the heat down. Just before serving, add a good measure of freshly chopped coriander . The sauce will keep in the fridge for a couple of days.