Place finely grated onion in a mixing bowl; add panko crumbs and stir it all together.
To the mixing bowl add ground beef, minced garlic, egg, ketchup, mustard powder, worcestershire sauce, salt, and pepper; use your hands to mix it all together until well combined.
Shape 4 patties into an oval, to resemble a “steak”, and set aside.
Heat 2 tablespoons olive oil in a large skillet and set it over medium heat.
Add the beef patties to the heated oil and cook for 5 to 6 minutes.Carefully flip them over and continue to cook for 5 to 6 more minutes. Patties are cooked through when internal temperature registers at 155˚F.
Remove patties from skillet and set aside; keep them covered.
Return skillet to the burner and add 1 tablespoon olive oil.
When oil is hot, stir in finely diced onions and cook for 1 minute.
Stir in garlic and then mushrooms; cook for 4 to 5 minutes, or until mushrooms are soft.
Add butter and swirl around to melt.
Sprinkle the top with flour and cook for about 30 seconds, stirring constantly.
Pour in the beef broth; mix and stir until everything is combined and there are no lumps from the flour.
Add the beef patties and any juices back to the skillet.
Continue to cook for 5 to 8 more minutes, or until gravy is thickened; stirring occasionally. If gravy starts to thicken too soon, add a bit of water or beef broth to the skillet.
Transfer steaks to a platter.
Taste the gravy for salt and pepper; adjust accordingly.
Spoon gravy over the Salisbury steaks and serve.