Preheat your oven to 350°F (175°C).

In a large skillet over medium heat, cook the ground beef until browned. Drain excess fat.

Add the chopped onion and sliced mushrooms to the skillet. Sauté until the onion is translucent and the mushrooms are tender, about 5 minutes.

In a large bowl, combine the cooked beef mixture, breadcrumbs, egg, garlic powder, Worcestershire sauce, salt, and black pepper. Mix until well combined.

Shape the mixture into oval patties and place them in a greased 9x13-inch baking dish.

In a separate bowl, whisk together the cream of mushroom soup and milk until smooth. Pour this mixture over the patties in the baking dish.

Sprinkle the shredded cheddar cheese evenly over the top.

Bake in the preheated oven for 30-35 minutes, or until the patties are cooked through and the cheese is bubbly.

Remove from the oven and let it cool for a few minutes before serving. Garnish with fresh parsley if desired.