In a large mixing bowl, blend ground beef, panko breadcrumbs, tomato sauce, egg, mustard, Worcestershire sauce, oregano, salt, and pepper. Mix gently with your hands until just combined to ensure the patties remain tender. Form this mixture into four equal patties, about ¾ inch thick, and set aside.

Heat 2 tablespoons of vegetable oil in a large skillet over medium heat. Once shimmering, add the patties, searing for 4-5 minutes on each side until browned and cooked through.

In the same skillet, add sliced onions and mushrooms, sautéing for about 5 minutes until softened and colored.

Pour in 1 ½ cups of low-sodium beef broth, stirring to combine with the onions and mushrooms. Add onion powder, garlic powder, and an additional splash of Worcestershire sauce, bringing to a gentle simmer.

To create a cornstarch slurry, mix 2 tablespoons of cornstarch with ¼ cup of water in a bowl. Stir this slurry into the gravy, which will thicken in about 1-2 minutes.

Once the gravy thickens, plate each patty and ladle the mushroom gravy over the top. Serve hot, ideally with mashed potatoes or green beans.