STEP 1: CHILE ARBOL - Preheat a large cast iron pan or skillet over medium heat. Separately, bring 1 cup of water to boil in a small saucepan. Toast the árbol peppers until they just begin to char, then remove from the pan. Add the árbol peppers to the boiling water, then turn off the heat and let soak 15-20 mins.
STEP 2: CHAR - Add the garlic and onion to the pan and cook until softened and blistered. Remove garlic when finished and add to molcajete. Add in the tomatillos and serranos and cook until blistered and softened.
STEP 3: GRIND - Grind the garlic until it turns to a paste, then add in the softened arbol peppers and grind. When the tomatillos, onions, and serranos are done cooking, add them to the molcajete and grind until your desired consistency. Finally, discard some of the thicker cilantro stems, then add the cilantro to the salsa and grind. Add salt to taste and serve.