Optional step: In a microwavable bowl, add 50 ml chicken stock and sprinkle in ½ tsp gelatin powder. Let it bloom for 5-10 minutes and then microwave for 30 seconds or in 20 second intervals until the gelatin is dissolved. Place in the refrigerator and leave to set for 3 hours. Once set, break up into small pieces using a spoon.
Place a frying pan on the stove and heat on medium. Melt 1 tsp unsalted butter and add 100 g yellow onion(s).
Once the onion is softened and slightly golden, add 100 g ground beef to the pan and fry until browned on the surface.
Pour in ½ tbsp mirin and 1 tbsp Worcestershire sauce. Stir and continue to fry until liquid is almost gone, then remove the pan from heat and allow the mixture to cool to room temperature.
In a small bowl, mix 3 tbsp panko breadcrumbs with 1 ½ tbsp whole milk.
Take a mixing bowl and add 150 g ground beef. Sprinkle with 1 pinch salt and pepper and roughly mix.
Add the wet panko mixture, 1 pasteurized egg yolk, ½ tsp tomato ketchup, 1 tsp sugar, 1 pinch nutmeg powder, and the cooled cooked meat and onions from earlier to the bowl. Mix well. (If using chicken stock jelly, add that too.)
Divide the mixture into equal portions (I shaped into 4) and shape into ovals/discs.
Place each patty on a plate, cover and rest in the fridge for 30 minutes.
Preheat your oil to 160 °C (320 °F).
Prepare 3 plates or containers. One with flour, one with whisked egg and one with panko breadcrumbs.
Coat each patty with 85 g all-purpose flour, then 1 large egg, then 150 g panko breadcrumbs. Be sure to press the breadcrumbs down gently without squashing the patty.
Coat each patty with egg and panko one more time.
Place the menchi katsu in the oil and fry for 2 minutes on each side. (Preferably cook 2 at a time.)
Remove from the oil and place on a wire rack. Increase the heat of the oil to 180 °C (356 °F).
Fry the menchi katsu in the hotter oil for 1-2 minutes each side or until golden.
Remove from the oil and place on a wire rack to allow the excess oil to drip off.
Mix the sauce ingredients in a small bowl or serving jug.
Serve menchi katsu with a sprinkling of dry parsley and the homemade sauce. Drizzle or dip and enjoy!