Preheat oven to the heat recommended in the frozen pie instructions and then add 10 degrees to temperature.
Peel potatoes and place in water and bring to the boil. Once boiled, mash the potatoes adding the butter and onion and milk – as needed - to get to your preferred consistency. Place big dollops of the mashed potato on top each individual frozen pie. Run a fork through the potato to give it presentation.
Place pies in oven and add 10 minutes to the pie cooking instructions to give the pies enough time to cook and give the potatoes a nice brown topping on top.