01 - Preheat oven to 400°F. Lightly grease a 9x13-inch baking dish with olive oil or cooking spray.
02 - In a blender or food processor, combine the tomatillos, ranch dressing, cilantro, jalapeño, garlic, lime juice, cumin, salt, and black pepper. Blend until completely smooth, scraping down the sides as needed.
03 - Place the chicken breasts in a large bowl or gallon-size zip-top bag. Pour half of the tomatillo ranch sauce over the chicken, reserving the remaining sauce for serving. Seal and marinate for at least 15 minutes, or up to 2 hours in the refrigerator for deeper flavor.
04 - Remove the chicken from the marinade and discard any used marinade. Transfer the chicken to the prepared baking dish. Drizzle with olive oil and season lightly with salt and pepper.
05 - Bake for 25 to 30 minutes, or until the chicken reaches an internal temperature of 165°F and the juices run clear when pierced with a fork.
06 - Remove from the oven and let the chicken rest for 5 minutes. Slice or serve whole, spooning the reserved tomatillo ranch sauce generously over the top. Garnish with additional fresh cilantro if desired.