In a medium mixing bowl, combine all ingredients for the beef steak and mix very well.
Divide the meat into 6 or 8 even balls and roll them into the patties. Place them on the parchment covered cutting board until ready to cook.(Pro tip: try to work as quickly as you can so the heat from your hands don't start melting the fats in the meat.)
In a small bowl, mix together beef stock, tomato paste, Worcestershire sauce, pressed garlic, Dijon mustard, thyme, salt, and pepper. Whisk well and set aside.
Preheat a large skillet, over medium heat, and add some oil. Place steak patties into the skillet and cook for about 5 minutes on each side.
Take the patties out and set aside.
Add butter into the pan and let it melt. Add in sliced onion and let it start sautéing until transparent. Stir in sliced mushrooms and cook until both are cooked through.
Sprinkle flour over the veggies and mix until evenly coated. Give the beef stock mixture another stir and start slowly pouring it into the pan, while carefully mixing. Keep stirring until everything is evenly combined.
Heat the gravy mixture through and once it starts to thicken, add the beef patties back into the pan. Bring mixture to a gentle simmer and lower the heat to medium-low.
Cover the pan with a lid and simmer for about 10-15 minutes, until patties are cooked through flipping the patties half way through. (You can cut into the thickest patty to make sure it's done.)
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Notes
*Gluten Free Notes: to make this recipe gluten free, simply use gluten free bread crumbs in the beef mixture and use gluten free all purpose flour to thicken the gravy.
Storing: Make sure to let the meat and gravy cool to room temperature prior to storing! Transfer everything into an airtight food storage container and store in the refrigerator. Properly stored, it should stay good for about 3-4 days.
Reheating: you can simply reheat individual portions in the microwave for 30 seconds at a time until heated through. Try not to overheat or the meat will get tough! As another option, you can reheat the Salisbury steaks in a pan over medium-low heat. Cover the pan with the lid as the steaks are reheating for the best results.