Grate the onion using a box grater directly into the breadcrumbs. Mix and let it soak for a few minutes.
In a large bowl, combine all the Salisbury Steak ingredients. Use your hands to mix until well combined. Mix for a few minutes until the mixture becomes somewhat "pasty," ensuring that the patties will hold together.
Divide the mixture into 5 equal portions and form them into oval-shaped patties, each around 3/4 inch (1.75 cm) thick.
Heat olive oil in a skillet over high heat. Add the steaks and cook for about 1 minute on each side until they are browned. They will still be raw inside. Remove them from the skillet.
For the Gravy:
If the skillet is looking dry, add a bit more oil. Add the chopped onion and garlic and cook for about 2 minutes until the onions become slightly translucent.
Add the sliced mushrooms into the skillet and cook for 2-3 minutes until they turn golden.
Turn the heat down to medium. Add butter. Once it melts, add the flour and cook for about 30 seconds, stirring constantly.
Gradually pour in the beef broth, stirring as you go. Once the mixture is mostly lump-free, whisk in the remaining Gravy ingredients.
Return the steaks along with the juices from the plate to the skillet. Cook for 5-7 minutes, or until the gravy thickens, stirring occasionally around the steaks. If the gravy thickens too quickly, add more water.
Taste the gravy and adjust the salt and pepper to your preference.
Serve the Salisbury steaks topped with the mushroom gravy, preferably over mashed potatoes. You can sprinkle a bit of parsley on top if desired.