Grate the half onion directly over the panko breadcrumbs in a large mixing bowl, allowing the breadcrumbs to soak in the moisture. Add ground beef, minced garlic, egg, ketchup, crumbled bouillon cube, Worcestershire sauce, and dijon mustard. Gently mix until just combined, being careful not to overwork the meat. Shape the mixture into five oval patties, each about ¾ inch thick.

Heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Once the oil shimmers, gently add the patties to the skillet. Sear them for about 1 minute per side, or until they develop a rich brown crust. The center will still be slightly raw; remove the patties from the skillet and set aside while you prepare the gravy.

In the same skillet where you cooked the patties, add the finely chopped onion and minced garlic. Sauté over medium heat for about 2 minutes, stirring frequently, until the onion is translucent and fragrant—this will build the flavor base for your savory mushroom gravy.

Next, add the sliced mushrooms to the skillet with the sautéed onions and garlic. Cook for about 2-3 minutes, stirring occasionally, until the mushrooms turn golden brown and reduce in size. This step adds an earthy flavor that beautifully complements the Salisbury Steak with Mushroom Gravy.

Lower the heat to medium-low and add 2 tablespoons of unsalted butter to the skillet, letting it melt. Whisk in 3 tablespoons of flour, stirring constantly for 30 seconds to create a roux. Gradually pour in the beef broth while whisking until smooth, then stir in the water, dijon mustard, and Worcestershire sauce. Cook for a few minutes until the gravy thickens.

Return the browned patties to the skillet, nestling them in the rich mushroom gravy. Cover the skillet and let them simmer for about 5-7 minutes, or until the patties are cooked through and the gravy has thickened to your desired consistency. Adjust salt and pepper to taste as needed.

Once done, carefully plate the Salisbury Steak topped with generous amounts of mushroom gravy. This dish is perfect served over creamy mashed potatoes or buttery egg noodles. Garnish with fresh parsley if you like a pop of color. Savor this homemade comfort food at your family table!