Remove the stems and seeds from the peppers. On a preheated comal or skillet at medium heat, dry roast the chiles, tomatillos and garlic. I would suggest roasting the dried chiles first, since they require your full attention. Toast them just until they become aromatic and change color slightly. This only takes a few minutes.
Using a metal spatula, press the peppers to help them roast a little faster. As soon as they become aromatic, transfer them to a pot of simmering water. Simmer for 10 minutes and let sit in water until cool.
On that same comal, dry roast the tomatillos and garlic for 15 minutes, turning as needed. Let cool for a minute, then remove the skins from garlic.
Using a slotted spoon, transfer the chile cascabel to the blender. Add all of the chile de arbol. Add 1 cup of water with salt to taste. Blend on high until very smooth. I blend for almost 1 minute, twice.
Add the garlic and tomatillos to the blender. Again pulse to blend, then blend on high until smooth. Season as needed.
Heat 3 tablespoons of grapeseed oil to medium heat. After a few minutes, add the salsa from the blender. Reduce heat to a slow simmer and cook for 7-10 minutes. Transfer to serving bowl and fold in the diced onions.