In a large bowl, combine the ground beef, breadcrumbs, egg, milk, Worcestershire sauce, garlic powder, onion powder, salt, and pepper. Mix gently until just combined. Shape the mixture into 6 to 8 oval patties.
Heat olive oil in a large skillet over medium-high heat. Sear the patties for about 2–3 minutes on each side until a nice brown crust forms (they do not need to be cooked through).
Place the sliced onions at the bottom of your slow cooker. Arrange the seared beef patties over the onions.
In a measuring cup, whisk together the beef broth, gravy mix, and Worcestershire sauce. Pour the mixture directly over the patties.
Cover and cook on LOW for 5 to 6 hours (or HIGH for 3 hours) until the beef is tender.
About 30 minutes before serving, gently remove the patties. Stir the cornstarch slurry into the bubbling liquid. Return the patties to the slow cooker, spoon the sauce over them, cover, and let the gravy thicken.
