Place a dry cast iron frying pan over medium heat, when hot, toast the chillies first inside then outside until just toasted and smelling fantastic.
Place in a bowl and cover with hot tap water, place a small plate on top of the chillies to keep them submerged, let soak for 20 minutes or so or until soft.
Using the same frying pan over medium heat place the tomatillos, onion and garlic. Roast the tomatillos on all sides until soft and black in places, flip the onion when it becomes chard and cooked, flip the garlic when black in places.
Remove the vegetables as they are cooked to a jug suitable for a stick blender. Drain and add the chillies, salt and sugar if using.
Blend until very smooth, scrape into a bowl and stir in the mezcal, reserve.