Prepare the potatoes: Peel and cut the potatoes into large cubes. Place them in a pot with 2 eggs and cook for 10 minutes. Meanwhile, finely chop the onion. In a skillet with a little oil, sauté the onion for 1 minute.

Cook the ground beef: Add the ground beef to the skillet with the onion. Season with salt and pepper to taste. Cook until the meat is nicely browned. Set aside when done.

Make the mashed potatoes: Transfer the cooked potatoes to a large bowl. Add the 2 hard-boiled eggs and mash everything together using a potato masher or a fork until smooth.

Combine meat and potatoes: Add the cooked meat to the bowl with the mashed potatoes. Mix well until evenly combined.

Shape the croquettes: Lightly moisten your hands, take a heaping spoonful of the mixture, and form into balls. Flatten them slightly to about 1/2 inch thick. Make sure all croquettes are the same size.

Coat the croquettes: Dredge each croquette in flour, then dip in 1 beaten raw egg, and coat thoroughly with panko breadcrumbs. Make sure each croquette is well coated.

Fry the croquettes: Heat oil in a pot to 300–320°F (150–160°C). Carefully place the croquettes in the hot oil and fry for 4 to 5 minutes, turning regularly until golden brown and evenly cooked.

Serve: Remove the croquettes from the oil and drain on paper towels. Serve hot, optionally with soy sauce.