Heat 2 tablespoons of oil in a skillet over medium heat. Add the finely diced onion and sauté until translucent, about 4 minutes. Next, introduce the minced garlic and cook further until fragrant, approximately 40-60 seconds. Allow the mixture to cool for 1-2 minutes.

In a large bowl, combine the cooled onion mixture, ground beef, panko breadcrumbs, egg, tomato sauce, yellow mustard, Worcestershire sauce, oregano, ground black pepper, and salt. Mix just until combined to avoid overmixing.

Form the mixed ingredients into 4-5 oval-shaped steaks.

In the same skillet, heat the remaining 2 tablespoons of oil over medium heat. Brown the steaks in batches for about 2 minutes (1 minute per side) before transferring them to a plate.

In the same skillet, add 1 tablespoon of vegetable oil and sauté the sliced onion until browned, roughly 5 minutes. Incorporate the sliced white button mushrooms and cook until they turn golden.

Return the steaks to the skillet, pour in the low-sodium beef broth, and season with onion powder, garlic powder, Worcestershire sauce, salt, and pepper. Bring to a boil, cover, and cook for 15 minutes.

In a separate bowl, mix the cornstarch with water or beef stock to create a slurry and gradually add this into the skillet. Cook for another 5 minutes, or until the gravy thickens slightly.

Ensure the steaks are done when the internal temperature reaches 160°F (71°C).

Plate the Salisbury Steak with a generous ladle of the savory gravy on top. Serve hot, ideally over creamy mashed potatoes or buttery egg noodles.