In a mixing bowl, combine the ground beef, breadcrumbs, milk, egg, half of the chopped onion, minced garlic, Worcestershire sauce, salt, and pepper. Mix until just combined.
Form the mixture into 4 equal patties.
Heat a skillet over medium-high heat. Add a little oil and brown the patties on both sides, about 2-3 minutes per side. Remove from skillet and set aside.
In the slow cooker, add the remaining chopped onion and sliced mushrooms. Place the browned patties on top.
Pour the beef broth over the patties and mushrooms.
Cover the slow cooker and cook on low for 6 hours, or until the beef is cooked through and tender.
If you’d like a thicker gravy, mix cornstarch with water in a small bowl and stir it into the gravy during the last 30 minutes of cooking.
Once done, serve the Salisbury steak with the gravy spooned over the top.
For added flavor, serve with mashed potatoes or rice.