After peeling and washing the tomatillos, transfer them to a pot and cover with water, To the pot also add the onion and serrano peppers without the stems. Bring up to a boil, reduce to a simmer and continue cooking for 7-10 minutes. Remove from heat.
In a small skillet, add the dried chile de arbol and the peeled cloves of garlic. Heat to just below medium. Add 1 tsp of oil. After a few minutes the ingredients will begin to sizzle and become aromatic. Toss in the hot oil for 2-3 minutes, then remove from heat.
Using a slotted spoon, transfer the tomatillos, serranos and onion to the blender. Add only the garlic at this time. Pour in 1/2 cup of the cooking water in. Add a big handful of freshly washed cilantro to the blender. Season to taste with salt. Blend on high until smooth. Now add the chile de arbol and pulse to blend. I like to see the red chile flakes in the salsa.
Transfer the blended salsa to a sauce pan and heat to medium. When it comes up to a simmer, reduce heat slightly and continue cooking for 7-10 minutes. Cooking the salsa will cook out the natural pectin and will yield a smoother salsa when refrigerated.