Mince the onion and shiitake mushrooms.

Mix ground beef and pork with salt and pepper in a bowl until sticky.

Add minced onion, shiitake mushrooms, breadcrumbs, and yakiniku sauce to the meat mixture. Divide into two portions and shape into oval patties with a slight indentation in the center.

Heat salad oil in a frying pan over medium heat and cook the patties until browned on one side.

Flip the patties and cook until browned on the other side. Cover with a lid and cook on low heat for 7 minutes until fully cooked. Remove from the pan.

Wipe excess oil from the pan with a paper towel. Add sauce ingredients (yakiniku sauce, sake, mirin, pepper, and water) and simmer over medium heat until thickened.