Finely chop the stewed shiitake mushrooms and the onion.

Sauté the onion well, and set aside to cool.

While the ground beef and pork blend is still cold, add the salt and pepper, and rub in well, until the mixture becomes thick and sticky with a whitish color.

Next, add the shiitake mushrooms, egg, onion, breadcrumbs, and milk, and mix well. Shape the mixture into oval-shaped patties, taking care to ensure that no air is trapped inside.

Heat a frying pan on high heat, and add vegetable oil. Fry the Hamburg steaks on medium heat. Once their color has changed, flip them. Cover and cook on low heat.

Once the Hamburg steaks are cooked, leave the juices in the pan, add the ketchup and Worcestershire sauce, and allow them to simmer for one minute on low heat.

Serve the Hamburg steaks on plates, and drizzle with the sauce made in the previous step.