In a small bowl, combine the mayonnaise, ketchup, chopped sweet pickles, and a pinch of salt. Mix well and set aside.
In a large mixing bowl, combine the leftover mashed potatoes, chopped corned beef, shredded Swiss cheese, sauerkraut, and sliced green onions. Mix until all ingredients are well incorporated.
In a separate bowl, whisk together the eggs and water until well combined.
Add the egg mixture to the potato mixture and stir until everything is evenly distributed.
Using your hands, scoop out a portion of the mixture and shape it into small patties or fritters, about 2 inches in diameter.
Spread the panko crumbs on a plate. Gently press each fritter into the panko crumbs, ensuring both sides are coated.
In a frying pan or deep fryer, heat about an inch of vegetable oil over medium heat.
Carefully place a few fritters in the hot oil, frying until golden brown, about 3-4 minutes per side.
Once cooked, transfer the fritters to a plate lined with paper towels to drain excess oil. Serve warm with the prepared dipping sauce on the side.