Add potato chunks to a medium-sized pot and fill the pot with enough cold water to cover the potato chunks. Bring the water to a boil, reduce the heat to medium, and continue to boil the potato for 15-20 minutes until they are fork-tender. Drain and set aside.

While the potatoes are cooking, heat 1 tablespoon of oil in a non-stick pan over medium heat. Add diced onions and sauté for about 2 minutes.

Add ground beef to the pan, break it into small pieces and sauté until browned. Season with 1/2 teaspoon soy sauce, 1/2 teaspoon sugar, 1/4 teaspoon salt, and 1/4 teaspoon ground black pepper. Set aside.

Add cooked potatoes, butter, 1/4 teaspoon (or to taste) salt to a large bowl. Mash the potatoes with a potato masher until smooth. Add the cooked ground beef to the bowl and mix until well combined.

Scoop out about 1/4 cup of mashed potato mixture and form it into a ball with your hands. Flatten the center of the potato ball then place about 2 tablespoons of shredded mozzarella cheese in the middle. Wrap the potato mixture tightly around the cheese with your hands, forming a 2” thick potato patty.

Place the flour, beaten eggs, and Panko bread crumbs in 3 separate shallow bowls. Season the flour with some more salt and pepper for more flavors. First, coat a potato patty with flour, then dip it in the beaten eggs, and lastly, evenly cover it with Panko crumbs on all sides.

In a medium-sized pot or deep skillet, heat 3-4 cups of oil, enough to cover the potato patties. Once hot, carefully place the potato patties in hot oil and fry for 2-3 minutes, until golden brown on each side. You may have to do this in batches.

Plate the beef croquettes (over some shredded green cabbage if desired). Drizzle some Tonkatsu sauce over each croquette.