1. Combine the ground beef, breadcrumbs, finely chopped onion, milk, egg, Worcestershire sauce, garlic powder, salt, and freshly ground black pepper in a mixing bowl. Mix until all of the ingredients are properly combined.
2. Form the mixture into oval or round patties 1/2 to 3/4 inch thick. This recipe makes 4-6 patties, depending on the size you choose.
3. Brown the beef patties on all sides in a pan over medium-high heat. This process is used to sear the patties and increase their taste. Put the browned patties in the slow cooker.
4. Whisk together the condensed cream of mushroom soup, dry onion soup mix, and beef broth in a separate basin until thoroughly blended. If you want a thicker sauce, add 2 tablespoons of all-purpose flour at this point.
5. If using sliced mushrooms, place them in the slow cooker with the browned meat patties. Then, drizzle the mushroom gravy over the patties and mushrooms.
6. Cover the slow cooker and simmer for 4-5 hours on low, or until the beef patties are soft and cooked through. Slow cooking enables the flavors to blend harmoniously.
7. When the Slow Cooker Salisbury Steak is done, serve it hot. For a splash of color and freshness, garnish with chopped fresh parsley.