Savvy Salisbury Steak

Ingredients

• 1 pound ground beef
• 1/2 cup breadcrumbs
• 2 tablespoons ketchup
• 1 tablespoon Worcestershire sauce
• 1 teaspoon mustard powder
• 1 egg
• salt, to taste
• black pepper, to taste
• 2 tablespoons butter
• 6 ounces mushrooms, sliced
• 1/2 large onion, sliced
• 2 1/2 cups beef broth
• 2 tablespoons Worcestershire sauce
• 2 tablespoons cornstarch
• 1 tablespoon water
• dried parsley, optional, to taste, for serving
• mashed potatoes, optional, to taste, cooked and hot, for serving