Boil potatoes until softened. Drain and mash potatoes while they are hot. Heat some oil in a medium skillet and saute onion and beef until cooked through. Mix mashed potatoes and onion and beef in a bowl. Season with salt and pepper and let it cool. Make flat and oval-shaped patties. Coat each piece with flour. Dip in beaten egg, and coat with panko at last. Deep-fry in about 350 degrees F oil until browned.