Preheat oven to 400 degrees. Line a large cooking sheet with parchment paper.
Placed mashed potatoes in a medium sized bowl. Set aside. Saute white parts of green onion in 2 tablespoons butter until softened. Add cooked onion to mashed potato. fold in.
Place grated onion in a separate bowl. Cover potato shreds with water. Stir to separate excess starch. Using a spoon with holes, remove and place shreds in a clean dish towel. squeeze towel over bowl allowing waterto drip back into bowl. Add squeezed out raw shreds to mash mixture. Stir. Gently pour off water and add remaining starch to mashed bowl. Fold in 1/2 cup milk.
In a separate bowl, whisk together 1/2 cup flour and baking powder. Add to potato mixture. Fold in eggs. Add salt to taste. Add a bit more reserved flour or milk as needed to create a thick batter.
In a large skillet, heat 3 to 4 tablespoons of butter over medium heat to melt. Drop 4 pancakes, about 1/4 cup of potato batter each, into melted butter. Fry gently, about 2 minutes per side, until browned. Repeat with the rest of the batter. Transfer cooked pancakes to prepared cooking sheet.
Bake at 400 for about 10 minutes until heated. Serve warm with sour cream and sprinkle with green onion as dsired.