Use kitchen scissors to slice the stem off the ancho chile, then cut it open vertically along one edge and spread it flat.
Heat a large, dry cast-iron skillet over medium heat. Add the chile and toast 1 to 2 minutes per side until fragrant. Transfer to a bowl, cover with water, and soak.
Return the skillet to medium heat. Add the whole tomatoes, onion wedges, and serrano. Cook 15 to 20 minutes, rotating as they brown, until the tomatoes and serrano are browned and blistered and the onion is charred and soft.
Drain and pat the chile dry. Place it in a blender with the charred tomatoes, onion, serrano, garlic, lime juice, avocado oil, and salt. Blend until smooth.
Add the cilantro and pulse until combined with visible green flecks. Season to taste with additional lime juice and salt.