In a large bowl, combine the ground beef, breadcrumbs, egg, garlic powder, onion powder, Worcestershire sauce, salt, and black pepper. Mix gently until just combined.
Divide the mixture into 6 oval hamburger steaks.
Heat the olive oil in a large skillet over medium-high heat. Brown the hamburger steaks for 2–3 minutes per side. They do not need to be fully cooked.
Place the sliced onions and mushrooms in the bottom of the slow cooker.
Arrange the browned hamburger steaks over the vegetables.
In a bowl, whisk together the beef broth, cream of mushroom soup, and brown gravy mix until smooth.
Pour the gravy mixture over the hamburger steaks.
Cover and cook on LOW for 6–7 hours or HIGH for 3–4 hours, until the patties are tender and cooked through.
Spoon the onion and mushroom gravy over the hamburger steaks and garnish with parsley before serving.