In a large mixing bowl, combine ground beef, Panko breadcrumbs, egg, ketchup, 1 tbsp Worcestershire sauce, 1 tsp Dijon mustard, onion powder, garlic powder, 1 tsp salt, and 1/2 tsp black pepper. Mix gently with your hands until just combined, being careful not to overmix.

Divide the beef mixture into 4 equal portions. Shape each portion into an oval-shaped patty, about 3/4-inch thick. Ensure the edges are smooth.

Heat 1 tbsp olive oil in a large skillet over medium-high heat. Once hot, carefully place the patties into the skillet. Cook for 4-6 minutes per side, or until well browned on both sides and cooked through to your desired doneness. Remove the patties from the skillet and set them aside on a plate.

In the same skillet (do not clean it, the pan drippings add flavor), reduce the heat to medium. Add 2 tbsp unsalted butter. Once melted, add the sliced cremini mushrooms and chopped yellow onion. Sauté for 5-7 minutes, stirring occasionally, until the mushrooms have softened and released their moisture, and the onions are translucent.

Sprinkle 2 tbsp all-purpose flour over the mushroom and onion mixture in the skillet. Cook, stirring constantly, for 1-2 minutes to toast the flour and create a roux.

Gradually whisk in the 2 cups beef broth, stirring continuously to prevent lumps from forming. Add the remaining 1 tbsp Worcestershire sauce, 1 tsp Dijon mustard, and fresh thyme. Bring the gravy to a simmer, then reduce the heat to low and let it thicken for 5-7 minutes, stirring occasionally. Season the gravy with salt and black pepper to taste.

Return the cooked Salisbury steak patties to the skillet with the gravy. Spoon the savory mushroom gravy over the steaks and let them simmer in the gravy for another 5 minutes to absorb the wonderful flavors and heat through.

Serve hot, garnished with fresh parsley if desired. Salisbury steak is traditionally served with mashed potatoes or egg noodles.