5 dried ancho chiles
3 dried arbol chiles
2 Roma tomatoes
4 medium garlic cloves, peeled
1/4 medium yellow onion
2 teaspoons kosher salt
1 tablespoon mezcal
Set a small sauté pan over medium-high heat, add the dried chiles, and toast, stirring often, until they become fragrant, about 3 minutes.
Set the chiles aside and add the tomatoes to the pan, roasting until the skins are evenly blistered, about 5 minutes. Set aside.
Bring a small saucepan of water to a boil and add the toasted anchos.
Simmer until the water starts to change color and the red of the chiles becomes more apparent, about 5 minutes. Remove the chiles and carefully take off the stems—wear latex gloves to protect your hands from the fiery oils. Reserve the water. Remove the stems from the toasted arbol chiles.
In a blender, combine the anchos, arbols, tomatoes, garlic, onion, salt, and 1 cup of the reserved chile water.
Blend on high until smooth, about 2 minutes. Add the measured mezcal.
Taste the salsa and add additional salt if you think it needs more. If it seems too thick, add more of the chile water until you get the consistency you like. Serve at once, or place in an airtight container and refrigerate for up to 3 days.