Mix the ground beef, ground pork, sautéed onion, egg, panko (soaked in milk), milk, Kewpie mayo, salt, pepper, and nutmeg.
Clap out the air, shape the mixture, sear, and then steam with a splash of water.
For the sauce, simmer the ketchup, Worcestershire sauce, and soy sauce in a pan, then pour over the meat patty.
