We often have a lot of extra gravy so I use it here so it won’t go bad. But if you don’t have enough to spare just add an extra egg to bind the filling together.
Leftover mashed potatoes have a tendency to get dry which makes it hard to spread them on the top of the pie. Heat them up in the microwave with some milk and stir the warmed mixture together until they’re soft and relatively smooth.
In a normal pie pan there’s usually a lip that the crust can rest on while it bakes so it doesn’t ‘slide down the sides. That doesn’t happen with a casserole dish so folding the edges over the filling helps the crust to not slide down the sides while the pie baked.