Cut potatoes in half lengthwise; cut each piece crosswise into 1/2 inch thick slices. Place with onion in a large, shallow foil-lined baking pan. Combine the next 7 ingredients; drizzle 1 tbsp. mixture over potato mixture & toss until evenly coated.
Discard giblets & neck from chicken. Rinse chicken under cold running water; drain & pat dry. Place chicken breast side up, in center of the pan, moving potatoes aside. Brush chicken, including cavity, thoroughly with soy sauce mixture.
Roast in 375 degrees F oven about 1 hour & 15 minutes, or until meat thermometer inserted into thickest part not touching bone registers 180 degrees F, brushing chicken with soy sauce mixture every 30 minutes & stirring vegetables. Remove from the oven; let chicken stand 10 minutes before carving. Serve with potatoes.