Add chicken stock and gelatin powder to a bowl. Let sit for 5-10 minutes, then microwave until dissolved. Chill until set.

Melt butter in a frying pan, then sauté onion until softened.

Add ground beef to the pan, stirring, until browned.

Pour in mirin and Worcestershire sauce, cook until liquid evaporates. Cool to room temperature.

Combine panko breadcrumbs with milk in a small bowl. Set aside.

In a bowl, mix ground beef, salt, pepper, panko mixture, egg yolk, ketchup, sugar, and nutmeg. If using jelly, fold it in now.

Divide mixture into equal portions and shape into patties.

Refrigerate patties covered for 30 minutes to firm up.

Heat cooking oil to 160 °C and prepare three plates with flour, whisked egg, and panko.

Coat each patty in flour, egg, and panko, pressing gently.

Repeat egg and panko coating for added crunch.

Fry patties in hot oil for 2 minutes on each side until golden.

Drain on a wire rack and increase oil temperature to 180 °C.

Fry katsu again until crispy and golden. Drain on a rack.

Mix sauce ingredients in a bowl until smooth.

Serve menchi katsu with sauce on the side for dipping.