Soak the cut sweet potato in lightly salted cold water for 10 minutes to stop it browning and remove surface starch. Drain, then steam over simmering water for 15 to 18 minutes, until a knife slides through easily.

Tip the cooked pieces into a dry pan over medium-low heat and mash them there, stirring for 2 to 3 minutes until the mash turns from glossy to matte and stiff. This drives off moisture and is what stops the korokke bursting. Beat in the butter while hot.

Cook the minced onion in the teaspoon of oil over medium heat for 5 minutes until soft with no color, then stir it into the mash with the salt and pepper. Cool to room temperature, then beat in the egg yolk.

Divide the filling into 8 portions and shape each into a flattened oval about 3 inches long and ¾ inch thick, pressing firmly so no air pockets remain. Chill on a tray for at least 30 minutes.

Set up three shallow dishes with flour, the beaten egg and water, and the panko. Coat each oval in flour and tap off the excess, dip in egg and let it drip, then press firmly into the panko, covering every face and edge. Chill another 15 minutes.

Heat 2 inches of oil in a heavy pot to 340 to 350°F. Do not go hotter, as the sugar in sweet potato scorches the panko before the inside heats through.

Fry 2 or 3 korokke at a time for 2 ½ to 3 minutes total, turning once, until deep even golden. Lift onto a wire rack, not paper towels. Let the oil return to temperature between batches.

Soak the shredded cabbage in ice water for 10 minutes and spin dry. Serve the korokke with a pile of cabbage, tonkatsu sauce poured over the korokke, and lemon wedges.