Peel and cube potatoes before boiling until soft, then drain and mash. While potatoes are cooking, add 1 tablespoon of vegetable oil in a skillet over medium, add the onion and cook until it is translucent, about 3–5 minutes. Add in the ground beef, stirring to combine with the onion, and cook the beef until it is done. Add soy sauce to the meat-and-onion mixture and set aside. Mix mashed potatoes and onion and beef in a bowl, season to taste with salt and pepper and let cool. Make oval-shaped patties. Coat each piece with flour. Dip in beaten egg and coat with panko, pressing the patties into the panko to make it stick. Deep-fry in about-350ºF oil until browned, drain on paper towels and serve.