Dice 1/2 onion into small pieces and thinly slice 3 tbsp of spring onion.

Heat butter and 2 tbsp cooking oil in a frying pan over medium heat.

Sauté the diced onion for 3 to 4 minutes until soft and translucent.

Add 200 g ground beef and cook for 5 to 6 minutes, breaking it apart until browned.

Season the beef mixture with salt and black pepper, then stir in the sliced spring onion.

Pour 600 ml hot water over 600 g instant potato flakes in a large mixing bowl.

Stir the potato mixture until smooth, thick, and firm enough to shape.

Let the mashed potato cool for about 10 minutes until safe to handle.

Fold the cooked beef filling into the cooled mashed potato until evenly combined.

Shape the potato mixture into 12 compact round or oval patties.

Chill the shaped patties in the refrigerator for 30 minutes until firm.

Place wheat flour, beaten egg, and 120 g potato flakes into three separate shallow bowls.

Coat each chilled patty in wheat flour, then dip it into the beaten egg.

Press each egg-coated patty into the potato flakes until fully covered.

Heat 950 ml frying oil in a deep frying pan to approximately 175°C / 350°F.

Fry the coated korokke in batches for 3 to 4 minutes until crisp and golden brown.

Drain the fried korokke on a wire rack and serve warm with tonkatsu sauce.