In a large bowl, thoroughly combine the ground beef, breadcrumbs, milk, 1 teaspoon Worcestershire sauce, parsley, 1/2 teaspoon mustard, 1/2 teaspoon salt, and black pepper. Form into 8 equally sized balls and flatten into patties. Set aside.

Pour beef broth into a large measuring cup. Whisk in the tomato paste, remaining 1/2 teaspoon mustard, beef bouillon base, and remaining Worcestershire sauce until smooth. Set aside.

Heat a large, deep pan or skillet over medium-high heat. Add 1 tablespoon butter to pan and swirl to coat. Add the beef patties, cook 3 to 4 minutes on first side until well-browned, and flip, taking care to scrape the browned bits from the bottom of the pan when you flip. Cook patties for 3 minutes on the second side or until the ground beef is cooked through in the center. Remove the patties to a paper-towel lined plate, cover to keep warm, and set aside.

Pour the grease from the pan and add the remaining 3 tablespoons butter to the pan. Melt and swirl the butter over medium high heat. Add the mushrooms and sauté for 4 minutes, stirring occasionally and scraping/deglazing the bottom of the pan. Stir in the garlic and cook for 1 minute. Sprinkle the flour over the mushroom mixture and continuously stir while cooking for 1 more minute.

Gradually stir in the broth mixture. Bring to a boil and cook for 1 to 3 minutes or until the gravy has thickened to your desired consistency. Reduce the heat, add the cooked beef patties back to the pan, and submerge in the gravy. Cook for another couple of minutes until they are heated through.