Peel and cut the Starchy Potatoes into even chunks, place them in a pot of salted water, and boil until very tender, about 15–20 minutes; drain well and return to the pot to steam-dry for a minute.
Meanwhile, finely dice the Onion. In a skillet heat a tablespoon of Neutral Oil over medium heat, sauté the onion until translucent, then add the Ground Beef and cook until browned; season while cooking with Salt, Black Pepper and a pinch of Nutmeg.
Mash the drained potatoes until smooth, then fold in the cooked beef and onion mixture; taste and adjust seasoning with salt and pepper if needed. Allow the filling to cool until it’s easy to handle.
With slightly wet hands, shape the mixture into oval or round patties about 2–3 cm thick (you should get roughly 8–10 depending on size). Chill the patties for 10–15 minutes to help them hold together, if desired.
Set up a dredging station with three shallow dishes: All-Purpose Flour in the first, beaten Eggs in the second, and Panko Breadcrumbs in the third.
Coat each patty by lightly dusting with flour, shaking off excess, dipping into the beaten egg, then pressing into the panko so they are evenly covered.
Pour enough neutral oil into a heavy skillet to come about 1/2–1 inch up the sides and heat over medium-high until shimmering. Fry the croquettes in batches, turning once, until deep golden brown and crisp, about 3–4 minutes per side. Transfer to paper towels to drain.
Thinly slice the Green Cabbage for a fresh bed, plate the korokke on top and serve immediately with Tonkatsu Sauce for dipping. Enjoy hot for the best texture.
