Get your comal or pan very hot. If your arbol chiles are not already toasted, arrange them in one layer in the hot pan and let them toast about 15 to 30 seconds -- not too long, or they will get bitter. Turn them over to toast the other side, then move them to the bowl of your blender.
Arrange the tomatoes, onion, tomatillos, and garlic cloves on the hot pan, cut side down. Let them char for a solid 5 to 10 minutes. Try to move them after 5 minutes with a metal spatula, and if they are still stuck to the pan, let them be another minute or two until they release. Once the cut sides of the tomatoes and tomatillos are charred, move them to the blender. Flip the onions and garlic to char a little on the other side.
When the onions and garlic are nicely charred a bit, move them to a cutting board. Peel the garlic and put the cloves in the blender. Roughly chop the onion and put that in the blender. Add the lime juice, a pinch of salt and about 1/4 cup of water. Puree, adding more water if you need to make a smooth, pourable sauce. Adjust for salt and serve.