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 on high until completely smooth, scraping down the sides as needed.
03 - Place the chicken breasts in a large bowl or resealable 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 at room temperature, or refrigerate up to 2 hours for deeper flavor.
04 - Remove the chicken from the marinade, allowing excess to drip off, and discard the used marinade. Arrange the chicken in the prepared baking dish. Drizzle with olive oil and season lightly with salt and pepper.
05 - Bake on the center rack for 25 to 30 minutes, until the chicken registers 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 each portion. Garnish with additional fresh cilantro if desired.