Cut the tops off the dried chiles. Remove the seeds and discard (or use for extra spice). Roughly chop the tomatoes and onion.
Fill a small pot with 3 cups of water. Bring to a boil over high heat. Remove from the heat when it begins to boil and add the dried chiles. Let sit for 10 minutes in the hot water.
Remove chilies from the water and put into a blender. Add tomatoes, tomato sauce, onion, garlic and salt. Blend on high speed until the salsa is smooth, about 1-2 minutes depending on the strength of your blender.
Cover and store in the fridge. Serve chilled or at room temperature.
Notes
This salsa is meant to be very spicy. If it's too spicy for you, add more tomato sauce to dilute the heat.If you're unsure about the heat factor, use half the peppers and remove all of the seeds. If that's not spicy enough use more the next time and use some seeds.But if you love very spicy foods, add those seeds! Don't deseed the chiles de arbol for the most significant spiciness.A little salsa goes a really long way here. Just a little drizzle over your tacos, eggs, etc. will add a lot of heat. You can always add more so don't add much in the beginning.This salsa has a thin, sauce-like consistency. To make it chunkier, blend it for less time, use less tomato sauce, or use more fresh tomatoes. The best way to make a chunkier salsa is to eliminate some of the water that the tomatoes produce when blended. Do this by removing their pulp before adding them to the blender. Another way to make it chunkier is to use a food processor to chop the ingredients rather than blend them.