Cook mushroom with butter if you have some.
Sauté onion on medium heat until slightly brown, cool

In a bowl, mix ground meat and panko together, add cooled onion

Add egg, salt and pepper, mix well.

Heat pan with some oil, fry it at medium heat with loose lid, when both sides are browned, reduce the heat a little bit.

Wipe off some oil from the pan, add sauce ingredients, cook some more until hamburg is throughly cooked. Add water if the sauce is getting too thick