Finely chop the onion. Fry the onion in a frying pan with oil until soft and lay aside to cool.
Add Shiitake powder, salt, and pepper to cold minced beef and pork, then knead it in thoroughly until it becomes sticky and slightly white.
Next, add the egg, onion, Panko breadcrumbs, and milk and mix thoroughly. Make an oval shape while forcing out the air pockets inside the hamburger.
Heat up the frying pan on high heat and add vegetable oil, then fry the hamburger at medium heat. Once it browns, flip it over, put on the lid, and cook at low heat.
Add the tomato ketchup and Worcestershire sauce to the frying pan with the leftover juices from frying the hamburger, and let it boil for 1 minute on low heat.
Plate up the hamburger and pour the sauce over it.