01 - Preheat oven to 400°F. Lightly grease a baking dish with olive oil or cooking spray and set aside.
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, scraping down the sides as needed.
03 - Place chicken breasts in a large bowl or resealable plastic bag. Pour half of the tomatillo ranch sauce over the chicken, reserving the remaining sauce for serving. Marinate for at least 15 minutes at room temperature, or refrigerate for up to 2 hours for deeper flavor penetration.
04 - Remove chicken from the marinade, discarding any used marinade. Transfer 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 internal temperature reaches 165°F when measured with a meat thermometer and juices run clear.
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.