Wash potatoes well. I leave the skin on. Slice onion and potatoes and layer in the bottom of crock pot. I usually put onion first and then the potatoes. Sprinkle half of the ranch dressing mix over this layer.
On top of the potato layer, place your chicken pieces. Adjust amount depending on how many you are expecting for dinner. Sprinkle them with the rest of the dressing mix. Add a generous pat of butter to the top of each breast piece.
Set crockpot on high for 4-5 hours. Check periodically to see if it needs the addition of a bit of moisture. If needed, add the chicken broth or stock.