Preheat oven to 425. In a mixing bowl: mix ranch, butter & salt/pepper. Cut aluminum foil into 4 squares, coat with cooking spray. Place potatoes in a bowl, and coat with 4tbs of ranch mixture, coat evenly. Place chicken on foil, coat with s&p. Divide potatoes evenly betwwen the portions, add onion if desired. Drizzle rest of butter mixture over chicken.