In a large bowl, combine the beef, egg, breadcrumbs, milk, garlic powder, onion powder salt and pepper. Mix thoroughly then shape into 6 patties.
Heat the olive oil in a large pan over medium high heat. Add the patties in a single layer and cook for 4-5 minutes on each side or until golden brown.
Place the mushrooms and onion in the bottom of a slow cooker. Add the patties on top.
Pour the gravy and beef broth into the slow cooker. Cover and cook on LOW for 4-6 hours.
Remove the patties from the slow cooker and place on a plate. Cover to keep warm.
Combine the cornstarch with the water and pour into the slow cooker. Turn the slow cooker to HIGH and cook for 3-5 minutes until sauce is thickened. Spoon the sauce over the patties. Top with parsley and serve.