Preheat the oven to 375 degrees F (190 degrees C). Line a large baking pan with foil.

Cut potatoes in half lengthwise, then cut each piece crosswise into 1/2-inch-thick slices. Mix soy sauce, olive oil, vinegar, garlic, oregano, rosemary, and pepper in a small bowl.

Place potatoes and onion into the prepared baking pan. Drizzle 1 tablespoon soy sauce mixture over vegetables and toss until evenly coated.

Discard giblets and neck from chicken. Rinse chicken under cold running water; drain and pat dry. Place chicken, breast-side up, in the center of pan, moving vegetables aside. Brush chicken, including inside the cavity, thoroughly with soy sauce mixture.

Roast in the preheated oven until no longer pink at the bone and the juices run clear, about 1 hour 15 minutes, brushing with soy sauce mixture and stirring vegetables every 30 minutes. An instant-read thermometer inserted into the thickest part of the thigh, near the bone, should read at least 165 degrees F (74 degrees C). Remove from the oven and let chicken stand for 10 minutes before carving.