Combine the ingredients for the steaks in a bowl and mix completely. Divide the mixture into 4 equal parts, then shape the meat into oval size patties, about 3/4" thick.
Heat a large skillet over medium-high heat and add in 1 tbsp oil in pan. Sear the meat patties on each side for about 3 minutes, until a nice golden brown crust is formed on each side. (NOTE - the patties will not be cooked through at this point). Remove from skillet and set aside.
For the Mushroom Gravy:
In the same skillet, toss in the onions and cook until softened and translucent, about 5 minutes. Stir in mushrooms and cook until browned, another 3 minutes. Finally stir in the garlic and cook until fragrant, about 30 seconds.
Melt the 2 tbsp of butter into the onions and mushrooms then stir in the flour. Cook until the mixture is thick, about 1 to 2 minutes.
Reduce heat to medium low and gradually stir in the beef broth, until the mixture becomes smooth. Season with the Worcestershire sauce, salt and pepper and bring to a simmer.
Return steak patties to the pan and nestle into the gravy. Cover and cook about 10 minutes, until the internal temperature of the steak patties reaches 160 degrees F.
Serve salisbury steak warm over mashed potatoes with the mushroom gravy spooned over top. Enjoy!