Prepare the Patties: In a mixing bowl, combine the ground beef, egg, minced onion, breadcrumbs, crushed garlic, salt, and pepper. Mix until evenly combined.

Sear the Patties (optional): For extra flavor, sear the patties in a skillet over medium-high heat for 2-3 minutes on each side until browned. This step is optional but adds a deeper flavor to the dish.

Assemble in Slow Cooker: Place the sliced onion and mushrooms at the bottom of the slow cooker. Lay the beef patties on top of the vegetables.

Make the Gravy: In a separate bowl, whisk together the beef broth, brown gravy mix, ketchup, and Dijon mustard (if using). Pour the gravy mixture over the patties in the slow cooker.

Cook: Cover and cook on low for 4-5 hours or high for 2-3 hours, until the patties are cooked through and tender.

Thicken the Gravy: Thirty minutes before serving, mix the cornstarch with 2 tablespoons of water to create a slurry. Pour the slurry into the slow cooker and stir gently. Cover and let the gravy thicken as it finishes cooking.

Serve: Serve the Salisbury steaks with a generous spoonful of mushroom gravy on top. This dish pairs perfectly with mashed potatoes, rice, or vegetables.