Fill a 3 quart sauce pan with water and bring to a boil. Boil the tomatoes for 10 minutes, or until the flesh is peeling away and the tomato is tender.
In a frying pan, heat cooking oil until warm and place the chiles in the pan - watch them carefully, they can burn very quickly when the oil is hot.
Place the tomatoes, onion, garlic, chiles, and cilantro in a food processor. Pulse until the tomatoes are liquid, then blend until the chiles are ground and everything is well combined. Taste the salsa and season with salt to your liking. If necessary, add water and mix to desired consistency.
Dried chiles de Arbol can be found in most Mexican markets.