Patty Preparation: Combine the ground beef, egg, minced onion, breadcrumbs, garlic, salt, and pepper. Form into small patties, about 3/4 inch thick.

Slow Cooker Setup: Lay a bed of the sliced onions and mushrooms at the bottom of your slow cooker.

Brown the Patties: Although optional, browning the patties in a skillet on both sides before adding them to the slow cooker is recommended for enhanced flavor. Place the browned patties over the onion and mushroom layer.

Create the Gravy: Mix the beef broth, gravy mix, ketchup, and Dijon mustard (if using) in a separate bowl. Pour this mixture over the patties in the slow cooker. Cover and set to cook on low for 5 hours.