Line a tray or baking sheet with wax paper; set aside.
In a large bowl, use your hands or a fork to gently combine the ground beef, egg, cracker crumbs, Worcestershire sauce, salt, onion powder, and black pepper. Shape the meat mixture into 4 equal patties.
Place the patties on the prepared tray. Cover with plastic wrap and refrigerate for at least 1 hour, or overnight.
Heat 1 tablespoon of butter and 1 tablespoon of oil in a large skillet over medium-high heat. Fry the patties until browned and crispy on the outside and cooked through on the inside, about 3-4 minutes per side. Remove the patties to a plate and tent loosely with foil to keep warm while you make the gravy.
Add 1 more tablespoon of butter to the drippings in the skillet. When the butter melts, add the mushrooms to the skillet and fry until tender, about 5-7 minutes. Add the garlic and cook, stirring constantly, until fragrant, about 30 seconds.
Stir in the condensed soup and the milk; bring the mixture to a gentle simmer. Reduce the heat to low, and simmer the gravy for about 5 minutes, stirring frequently. Stir in the parsley.
Return the patties to the skillet. Warm over low heat, spooning the gravy over the patties.
Garnish with additional fresh parsley and serve immediately.