Place sliced onions and mushrooms into a large, heavy pan with vegetable oil, and cook on medium heat until onions are translucent and tender, about 15 minutes.
While the onions and mushrooms are cooking, prepare the hamburger patties. In a large bowl, add the ground beef, egg, breadcrumbs, minced onion, milk, seasoned salt, and garlic powder. Combine well until thoroughly mixed together.
Evenly portion out the ground beef mixture, shape into balls, then press into patties. Indent the center of each patty with thumb or finger.
Once the onions and mushrooms are cooked, remove from pan and set aside.
Place the raw patties into the pan, brown over medium heat for 5 minutes per side.
Remove patties from pan, and drain off all but 1-2 tbsp of grease.
Prepare the gravy. To the pan, add the gravy mix, beef broth, parsley, ketchup, and mustard. Stir well. Add the cooked onions and mushrooms.
Place the patties into the pan with the gravy ingredients, and cook on medium for 12-15 minutes, until patties are thoroughly cooked and the gravy thickens.