Peel the potatoes and cut them into quarters. Cook them in boiling water until they are soft. In a large bowl, mash the potatoes thoroughly.
Finely chop the onion. Heat the butter in a pan, then add the chopped onion and cook for about a minute. Next, add the frozen vegetables and cook for a couple of minutes. Season with salt and pepper to taste.
Combine the mashed potatoes with the vegetable mixture, and add additional salt and pepper to taste. Let it cool.
Divide the potato mixture into 8 equal portions and shape each portion into oval patties. Refrigerate the patties for an hour to firm them up.
Coat each potato patty with flour, then dip it into beaten eggs, and finally coat it evenly with Panko breadcrumbs.
Heat the deep frying oil to a temperature between 350-375°F (175-190°C), and fry the patties for 3 minutes on each side, or until they are golden brown and crispy.