To make the salsa, heat a griddle or heavy-based frying pan (or you can do this on the BBQ). Griddle the jalapeños until blackened on all sides, then drop onto a large sheet of foil and seal to make a parcel. Do the same with the onion, garlic and tomatoes, one by one, adding to the foil parcel (then resealing to keep the heat in) as they are charred. Let everything cool completely then de-stalk the chillies, remove the skin from the garlic cloves and cut the cores out of the tomatoes (and discard) and roughly chop. Put everything in a blender with the sugar, coriander and lime juice, and whizz until blended.