In a large bowl, combine the ground beef, panko breadcrumbs, tomato sauce (or ketchup), egg, yellow mustard, Worcestershire sauce, oregano, black pepper, and salt. Mix until just combined. Shape the mixture into 4 patties.
Heat 2 tbsp of vegetable oil in a large skillet over medium heat. Add the patties and cook until browned on both sides, about 4-5 minutes per side. Remove the patties from the skillet and set aside.
In the same skillet, add the sliced onions and mushrooms. Cook until softened, about 5 minutes.
Add the beef broth, onion powder, garlic powder, and Worcestershire sauce. Bring to a simmer and return the patties to the skillet. Cook the patties in the broth for about 10 minutes, or until cooked through.
In a small bowl, mix the cornstarch with water to create a slurry. Stir the slurry into the gravy to thicken it, and cook for an additional 1-2 minutes.
Season with salt and pepper to taste and serve the Salisbury steaks with the mushroom gravy on top.
