01 - Preheat oven to 400°F. Lightly grease a baking dish with olive oil or nonstick spray.
02 - In a blender or food processor, combine tomatillos, ranch dressing, cilantro, jalapeño, garlic, lime juice, cumin, salt, and black pepper. 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 remaining sauce for serving. Marinate for at least 15 minutes or up to 2 hours in the refrigerator for deeper flavor.
04 - Remove chicken from the marinade and discard any used marinade. Arrange the chicken in 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 and 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.