I usually start by wiping off the Chile de Arbols with a damp paper towel. De-stem and de-seed the chiles, but don't worry about getting rid of every last seed. Note: it's best to use caution when handling hot chiles. Be sure to wash your hands after handling them or you can use gloves.
I roast the chile pieces in the oven (400F) for 1-2 minutes until warm and fragrant. Alternatively you can flash roast them on a dry skillet.
Add the roasted chili pieces to a bowl and cover them with the hottest tap water you've got. Let them reconstitute for 20-30 minutes. If they float to the surface you can use a small plate or bowl to keep them submerged.
Husk the tomatillos and then rinse both the tomatoes and tomatillos. I usually remove the stems from both knowing that any juices leftover in the roasting pan will go into the blender.
Roast the tomatoes and tomatillos underneath the broiler for 5-7 minutes per side or until some char is forming. You can cut them in half for a quicker roasting time, just be sure to pour all the leftover juices in the blender.
Add the roasted tomatoes and tomatillos to a blender along with 1/3 small onion, 1 peeled garlic clove, 3-4 drained Chile de Arbols, and a pinch of salt. Combine well.
Taste for seasoning, adding more Chile de Arbols if you want more heat. I used 8 for this batch and it was ultra fiery!
Serve immediately and store leftovers in an airtight container in the fridge.