Get a pot of water up to a boil, enough to cover the tomatoes. Once the water reaches a boil, place the tomatoes inside. Prepare a large bowl with ice water.
Place the chile de arbol in a smaller bowl. Take some of the boiling water from the pot of tomatoes and pour onto the chiles. Cover the bowl and set aside. After the tomatoes have been in the boiling water for approximately 5 minutes, remove them and place into the large bowl with ice water and let them chill for about 10 minutes.
At this point, the skin from the tomatoes should easily peel off. Remove the stem portion and as much of the skin as possible from all of the tomatoes and put into a blender. As you place them in the blender, break the tomatoes into smaller pieces. Add the onion which should be cut into smaller pieces. You may need to do this in parts depending on how many tomatoes you have and how big your blender is. Puree the tomatoes for about 10 seconds. Add the spices.
Remove the chile de arbol from the bowl of hot water and place into blender with the tomatoes and spices. Blend well for at least 2 minutes. Remove from blender and place in covered bowl and let sit in refrigerator for at least an hour. The salsa is ready to enjoy now but it takes at least an hour for the flavors to properly marry.
You can make this more mild by adding less chile de arbol, or hotter by adding more. You can also add other peppers like habanero or ghost.