Soak the peppers in enough hot water to cover them until they are soft.
Fry the onion and tomatillos in the lard until they are fragrant. Add the water and broth to them and poach for 10-15 minutes.
Using a blender, food processor, or food mill, grind all the ingredients into a smooth paste. Use as much hot (boiled) water as required to achieve a thick milkshake texture.
Salt to taste
When this is made, prepare a cupcake tray with paper liners (the liners prevent flavour transfer to the metal).
Fill the cups with sauce then freeze.
After they are removed from the cupcake pan, store them in ziplock freezer bags. This gives you about 20 portions.
Notes
I render my own lard from the skin of double smoked bacon that my butcher sells and use the broth from that as well in the blender.