Mix and shape. Line a tray with parchment or wax paper. In a large bowl, gently combine the ground beef, egg, cracker crumbs, Worcestershire sauce, salt, onion powder, and black pepper with your hands or a fork, mixing just until combined. Shape into 4 equal patties, about 3/4 to 1 inch thick, and place on the tray.
Chill. Cover the patties and refrigerate for at least 1 hour, or overnight, so they firm up and hold together.
Sear. Heat 1 tablespoon butter and 1 tablespoon oil in a large skillet over medium-high heat. Fry the patties until browned and crisp on the outside and cooked through (160°F), about 3 to 4 minutes per side. Remove to a plate and tent loosely with foil.
Cook the mushrooms. Add the remaining 1 tablespoon butter to the drippings. Add the mushrooms and cook until tender, 5 to 7 minutes. Add the garlic and cook until fragrant, about 30 seconds.
Make the gravy. Stir in the condensed soup and milk and bring to a gentle simmer. Reduce heat to low and simmer about 5 minutes, stirring often. Stir in the parsley.
Combine. Return the patties to the skillet and warm over low heat, spooning the gravy over the top.
Serve. Garnish with additional parsley and serve immediately over mashed potatoes, noodles, or rice.