Mix the ground beef, panko, soup mix, black pepper, and Worcestershire sauce in a large bowl. Combine with your hands until fully incorporated.

Form into four ½-inch thick hamburger patties.

Using your thumb, press down the center of each patty to create an indentation.

Heat a large skillet over medium-high heat. Add 2 tablespoons of butter.

Add hamburger patties and cook for 5 minutes on each side. Transfer to a plate.

Reduce the heat to medium, add onions and mushrooms to the skillet. Saute for 5-7 minutes.

Whisk in the broth and bring to a boil.

Make the cornstarch slurry by mixing 2 tablespoons cornstarch with ¼ cup water.

Pour the slurry into the sauce. Stir for 2 minutes, or until the sauce has thickened.

Turn off the heat and stir in the remaining 2 tablespoons of butter.

Add the hamburger patties back to the skillet. Garnish with parsley and serve.