01 - Preheat oven to 400°F. Lightly grease a baking dish with cooking spray or oil.
02 - Combine tomatillos, ranch dressing, cilantro, jalapeño, garlic, lime juice, cumin, salt, and black pepper in a blender or food processor. Blend until completely smooth.
03 - Place chicken breasts in a large bowl or zip-top bag. Pour half of the tomatillo ranch sauce over the chicken, reserving the remainder for serving. Marinate for at least 15 minutes or up to 2 hours for deeper flavor.
04 - Remove chicken from the marinade and transfer to the prepared baking dish, discarding any used marinade. Drizzle with olive oil and season lightly with salt and pepper.
05 - Bake for 25 to 30 minutes until the chicken reaches an internal temperature of 165°F and juices run clear.
06 - Remove from the oven and let rest for 5 minutes. Slice or serve whole, spooning the reserved tomatillo ranch sauce over the top. Garnish with fresh cilantro if desired.