I have done 2 ways- Add sauted onion and then plop into the saute pan to let get crisp like hash browns OR Add the sauted onion and an egg and put into a lightly greased casserole dish and serve for dinner as a souffle type potatoe- no one ever realizes they were left overs.
I do almost the same as other posters- but I use only the egg yolk, and I dredge the potato patty in freshly grated breadcrumbs before frying. Last weekend, I also had some leftover shrimp ( from shrimp cocktail on TD), and added the shrimp, too. Made a nice snack while watching the football games.
Mine are more like potato cakes than pancakes but I love them for breakfast. I take the leftover potatoes and mix them with sharp grated cheese, scallions and some salt & pepper, form into cakes and pan fry in olive oil. I serve them with sour cream.
I do exactly the same, but I also add an egg or two (depending on the volume) to the mix. I regularly make more mashed potatoes than I need so I can make these the next morning.