In a mixing bowl, combine the ground beef, egg, breadcrumbs, garlic powder, salt, pepper, and 2 tablespoons of ketchup. Mix gently until just combined. Shape into 6-8 oval patties.

In your slow cooker, arrange the beef patties in a single layer. Slice the onions and mushrooms, distributing them over the patties.

In a separate bowl, whisk together the condensed mushroom soup, beef broth, Worcestershire sauce, and Dijon mustard until smooth. Pour over the patties and vegetables.

Cover and cook on low for 6-8 hours or on high for 3-4 hours until beef is cooked through and vegetables are tender.

For thicker gravy, combine cornstarch and water in a small bowl. About 15-20 minutes before serving, stir into the gravy and turn heat to high until thickened.

(Optional) Spoon extra ketchup over the patties before serving for added flavor.

Ladle the Salisbury Steak and gravy over mashed potatoes, rice, or egg noodles for serving.