Taste the mashed potatoes to make sure they are seasoned well with sea salt. If needed, add some salt. In a large bowl, mix together the cold mashed potatoes, grated cheeses and the egg. Form the dough into eight balls then press the balls down to form patties. Dip each of the cakes in the flour so they are lightly coated.Heat a splash of oil in a medium-sized frying pan over medium-high heat. Once the pan is hot, add half of the leftover mashed potato cakes. Let the cakes fry for 3-4 minutes, or until the bottoms are brown. Carefully flip the cakes over and fry until the other side is brown, too. Repeat with the remaining cakes.Serve the leftover mashed potato cakes with some sour cream and green onions on top.