Preheat the grill to medium-high heat.
In a bowl, combine the ground beef, diced onions, and mushrooms. Mix well.
Season the mixture with salt and garlic powder.
Shape the mixture into patties, ensuring they are evenly sized and compact.
Cook the hamburger steaks on the grill for about 4-6 minutes per side, or until they reach your desired level of doneness. Use a meat thermometer to check the internal temperature. Consider placing cheese slices on top of the steaks while they cook until they’re melted.
Remove the hamburger steaks from the grill and let them rest for a few minutes before serving.