Heat a large cast iron skillet or comal over very high heat. Once the pan is smoking hot, place the chilis on in a single layer (work in batches) and toast for a few seconds on each side. The chilis should turn bright red and become pliable, but be careful not to let them burn. It can happen in a matter of seconds and will turn the chilis bitter.

Transfer the chilis to a bowl and cover with about 2 cups of boiling water, or more as needed. Set aside and allow to soak for at least ten minutes. While the chilis are soaking, place the garlic cloves and whole piece of onion on the hot skillet, and turn until charred on all sides, about 8 minutes. Allow the garlic to cool, then remove the skins. Transfer the garlic, onion, chilis, about half of the water, vinegar and salt to a blender.

Blend on high until nice and smooth, then taste and add more water, vinegar and salt as needed to adjust the seasoning and thickness. Continue blending until totally smooth. Transfer to a bowl or squeeze bottle and keep refrigerated** for up to 2 months.