Start by preparing the mashed potatoes. Peel the potatoes and cut them into even-sized chunks, about 1 to 1.5 inches in size.

In a medium pot, cover the potato chunks with cold water, ensuring they're submerged. Add a pinch of salt. Place the pot on the stove over medium-high heat and bring to a boil. Reduce heat to a simmer and cook until the potatoes are tender when pierced with a fork, about 15-20 minutes.

Once cooked, drain the potatoes in a colander. Return them to the pot. Add butter to the hot potatoes and let it melt. Begin mashing the potatoes using a potato masher. Gradually add milk while continuing to mash until you achieve your desired consistency. Season with salt and pepper to taste. Set aside,

While the potatoes are cooking, you can begin to make the hamburger steaks. Mix the ground beef, panko breadcrumbs, soup mix, black pepper, and Worcestershire sauce in a large bowl.

Combine the mixture using your hands until well mixed. Form the beef mixture into four ½-inch thick hamburger patties. Press down the center of each patty lightly to create an indentation, using your thumb or the back of a spoon.

Reduce the heat to medium, and add the onions and mushrooms to the skillet. Saute for 5-7 minutes. Whisk in the broth and bring this onion gravy to a boil.

In a separate small bowl, 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.