Preheat the grill. Put the tomatillos, garlic cloves, onions and chillies on a baking sheet and cook for 5 mins on the rack closest to the grill until blackened. Remove from the oven, then peel the garlic cloves and squeeze into a food processor or blender. Add the rest of the charred vegetables from the baking sheet, along with 1 tsp salt, the chipotle peppers, coriander leaves and lime juice. Blend until smooth, then taste and add more salt, if needed.