One of my favorite things to do with leftover mashed potatoes is to make them into patties. My mother used to make these. You can add Parmesan cheese to the bread crumbs, herbs, spices, etc.
Mashed Potato Patties
2 cups mashed potatoes, cooked and chilled
1 egg, beaten
½ cup bread crumbs
2 Tbs olive oil
2 Tbs butter
Put the egg in a small dish, beat.
Put the bread crumbs in another dish.
Form patties out of the mashed potatoes. Dip them into the beaten egg, then the bread crumbs.
Sauté them in the olive oil and butter, over medium heat.
Cook patties until lightly brown and then flip to cook the other side. Repeat steps until mixture is all used up.
Sprinkle potato patties with your desired toppings.
Note – You can make the patties in advance. When you want to eat them, simply put them on a slightly greased baking tray, and put them in the oven at 350˚ for about 10 minutes.