2) Preheat the grill. Put the quartered tomatoes, sliced onion, and whole garlic cloves onto a roasting tray, spreading out evenly. Drizzle with plenty of olive oil and season well with salt and pepper and sprinkle with coriander sprigs. Grill until everything is nicely charred, about 10 minutes (you want lots of deep rich color so don't be afraid if some of the edges get pretty black).
3) Add the chilli mixture to a blender and puree. Remove the tomato/onion mixture from the roasting pan and carefully add it to the blender, (it will be hot). Blend until smooth (you may need to work in 2 batches). Once everything is pureed, pour the mixture back into the pot over low heat adding a little water if the salsa is too thick. Stir in the sugar and lime juice and season with salt and pepper, to taste. Transfer to a large serving bowl and serve.