Add all Salisbury steak ingredients to a bowl and mix by hand, folding gently in a circular motion.
Place beef mixture in refrigerator for one hour.
While beef mixture rests, prep & measure your gravy ingredients: dice the onion, slice the mushroom, cook your beef broth (if using boullion cubes), and prepare any side dishes (like mashed potatoes).
Divide beef into 4 (or 5) football-shaped patties, one inch thick in the middle.
Heat oil in skillet over high heat. Add the steaks and cook one minute per side, just until browned. Remove onto plate.
Reduce heat to medium, adding more oil. Cook chopped onion for 2 minutes or until translucent.
Add sliced mushroom and cook for 3 minutes, until moist and golden. Add butter.
Once butter is melted, slowly stir in all the Flour and cook about a minute.
Slowly stir in beef broth, then Worcestershire sauce, salt & pepper. Add the steaks & cook about 6 minutes, stirring gravy occasionally.