Use kitchen scissors to slice the stem off the ancho chile, then cut it open vertically along one of its edges. Spread it open so that it lays flat.

Heat a large, dry cast-iron skillet over medium heat. Add the chile and toast for 1 to 2 minutes per side, or until fragrant. Remove and transfer to a medium bowl. Cover with water and set aside to soak.

Return the skillet to medium heat. Add the whole tomatoes, onion wedges, and serrano and cook until the tomatoes and serrano are browned and blistered and the onion is charred and soft. Rotate the vegetables as they brown on each side, removing them from the pan as they are ready. The entire process should take 15 to 20 minutes.

Drain the chile and pat it dry. Place in a blender and add the charred tomatoes, onion, serrano, garlic, lime juice, avocado oil, and salt. Blend until smooth. Add the cilantro and pulse until combined. Season to taste.