20 whole serrano peppers (or 10 Serrano del Sol)
2 tablespoons chopped white onion
1 clove garlic, chopped
4 tablespoons water, cold
Salt, to taste
Toast the chiles on a medium-hot comal or griddle, turning them from time to time until they are blistered and charred.
While they are still hot, grind them with the rest of the ingredients to a rough paste in a molcajete or in a blender.