Finely chop the onion. Put it in a heatproof bowl with a teaspoon of oil, cover, and microwave at 600W for about 1 minute. Let it cool completely.
In a mixing bowl, combine the panko and milk and let sit until the breadcrumbs soften.
Add the ground meat, cooled onion, salt, pepper, and nutmeg (or allspice). Knead with your hands until the mixture turns sticky and pale.
Divide in two and shape into oval patties, tossing between your hands to remove air. Press a shallow dimple into the center of each.
Heat a little oil in a frying pan over medium heat. Add the patties and sear until deeply browned underneath, then flip.
Once both sides are browned, reduce heat to low, cover with a lid, and steam-cook about 7 minutes until the center is firm and juices run clear.
Remove the hamburgs and keep warm. Into the same pan add the sake, ketchup, and Worcestershire sauce and simmer 1 minute until slightly thickened.
Turn off the heat and stir in the butter until melted and glossy.
Plate the hamburg, pour the sauce over the top, and serve with shredded cabbage, tomato, and hot rice.
