Add the ground beef, half of the onion soup mix, egg, breadcrumbs, Worcestershire sauce, garlic powder, and black pepper to a large bowl. Gently mix the ingredients together until they are evenly combined. Form the mixture into 6 oval-shaped patties.
Heat the olive oil in a large skillet over medium-high heat. Brown the patties for about 2 minutes on each side. They do not need to cook completely through because they will finish cooking in the crockpot.
Add the sliced onions and mushrooms to the bottom of the crockpot. Place the browned beef patties on top of the onions and mushrooms. In a bowl, whisk together the beef broth, brown gravy mix, remaining onion soup mix, ketchup, and Dijon mustard. Pour the gravy mixture evenly over the patties.
Cover and cook on low for 5 to 6 hours or on high for 3 to 4 hours. Carefully remove the cooked patties from the crockpot and place them onto a plate. Stir together the cornstarch and cold water in a small bowl until smooth.
Whisk the cornstarch slurry into the gravy in the crockpot. Cover and cook for another 10 to 15 minutes, or until the gravy has thickened. Return the patties to the crockpot and spoon the gravy over the top before serving.