In a large mixing bowl, stir together the ground beef, breadcrumbs, onion, tomato sauce, beaten eggs, mustard and garlic until the ingredients are evenly dispersed throughout the ground beef.
Separate the mixture into 5-6, 1-inch thick patties.
Add the cooking oil to a large skillet and heat over medium heat until it sizzles if you splash in a drop of water.
Place the patties in the hot oil and sear them until browned, about 2-3 minutes, on each side. You may need to sear them in two batches, depending on the size of your pan. An overcrowded pan will not allow the ground beef to brown well.
Remove the patties from the skillet and place on aluminum foil and wrap up to wait until you’ve made the gravy. This will keep the patties hot and may continue to cook them slightly. They do not need to be cooked through yet as you will add them back to the pan with the gravy.
In the same skillet, heat 1 Tablespoon oil over medium heat and then add in the sliced mushrooms.
Saute the mushrooms for 5-7 minutes or until they are beginning to take on a golden color and soften on the outside. They should still be firm inside and hold their shape.
Add in the gravy mix to the mushrooms and gradually whisk in the beef broth until smooth.
Add the patties back into the skillet with the mushrooms and gravy and continue heating over medium low heat, whisking the gravy as much as you can, until it begins to boil.
Continue to boil for 1-2 minutes or until the gravy has thickened and the patties reach 160 degrees Fahrenheit in the middle.
Turn off the heat, cover the pan with a lid and allow it to sit for 5 minutes for the gravy to finish thickening and cooking.
Serve garnished with minced parsley, optional.
To Store: Salisbury Steak should be cooled to room temperature before storing. You can store it in an airtight in the refrigerator for 3-5 days.
To Freeze: To freeze your Salisbury Steak, make sure it is cooled to room temperature and placed in a freezer-safe container. It can be frozen for 2-3 months.
To Reheat: Frozen Salisbury Steak can be thawed in the refrigerator or microwaved directly from a frozen state.
Use your hands to mix together the ground beef mixture. This may sound gross (and you definitely want to wash your hands well afterwards) but it is by far the easiest way to get well seasoned, equally mixed patties. You will be able to feel any lumps of unmixed ground beef while you are mixing and will be able to see when everything is fully combined.
Brown your patties in batches to make sure they each get a beautiful, brown sear. Unless you have a really large pan, it is likely that the patties will be pressed together if you put them all in at the same time. This will make it harder for them to get that beautiful brown color you want. Also, since they will be resting while you work on the mushrooms anyways, it is perfectly fine to brown them in batches. Just be sure to wrap the ones that are already seared in aluminum foil to keep them warm.
Do not be tempted to walk away while the mushrooms and gravy are cooking--you will get a better result if you stay near and stir them every minute or so. The same goes with the gravy. Once the powdered gravy mix is added, start whisking as you gradually pour in the broth. Keep whisking until all the liquid is added and then add in the patties but continue to whisk any areas of gravy that you can to avoid getting lumps in your gravy.