Grating Onions: Begin by placing the breadcrumbs in a bowl. Utilize a box grater to finely grate the onion directly over the breadcrumbs. Mix the grated onion and breadcrumbs together using your fingers and allow them to soak for a few minutes.
Mixing the Ingredients: In a large bowl, combine the remaining ingredients for the Salisbury Steak. Use your hands to thoroughly mix the ingredients until they are just incorporated. Continue mixing for a couple of minutes until the mixture takes on a slightly "pasty" consistency, ensuring the patties hold together well.
Forming the Patties: Divide the mixture into 5 equal portions and firmly shape them into oval patties, approximately 3/4" or 1 2/3 cm thick.
Browning the Patties: Heat oil in a skillet over high heat. Add the patties and cook the first side for about 1 minute, or until it's nicely browned. Carefully flip the patties to brown the other side, bearing in mind that they will still be raw inside. Transfer the browned patties to a plate.
Cooking Aromatics: If the skillet seems dry, add a small amount of additional oil. Add the chopped onion and garlic and sauté for approximately 2 minutes, or until the onions become slightly translucent.
Sautéing Mushrooms: Introduce the sliced mushrooms into the skillet and sauté for 2 to 3 minutes, or until they acquire a golden hue.
Preparing the Gravy Roux: Reduce the heat to medium, add the butter, and let it melt. Once melted, incorporate the flour and cook for 30 seconds, stirring constantly.
Creating the Gravy: Gradually pour in the beef broth, stirring as you go to avoid lumps. Once the mixture is mostly free of lumps, whisk in the remaining ingredients for the gravy.
Completing the Steak Cooking: Reintroduce the cooked Salisbury Steaks along with any accumulated juices back into the skillet. Cook for 5 to 7 minutes, or until the gravy thickens, stirring occasionally around the steaks. If the gravy thickens too rapidly, add additional water as needed.
Seasoning the Gravy: Remove the steaks from the skillet onto a plate. Taste the gravy and adjust the salt and pepper to your preference.
Serving Salisbury Steaks: Plate the Salisbury Steaks, generously spooning the mushroom gravy over them. For an ideal pairing, serve them atop creamy mashed potatoes and garnish with a sprinkle of fresh parsley, if desired. Enjoy!