Combine ground beef, breadcrumbs, grated Parmesan cheese, egg, garlic powder, salt, and pepper in a mixing bowl. Mix until just combined and form into four equal-sized patties.
In a large skillet over medium-high heat, add olive oil and let it heat up before placing in the formed patties. Sear each side for about 4-5 minutes until they develop a brown crust; then remove from the skillet and set aside.
In the same skillet, add sliced onions and mushrooms, stirring frequently until they soften and become golden brown, about 6-8 minutes.
Pour in beef broth and Worcestershire sauce, scraping up any brown bits from the bottom of the pan. Bring to a simmer before returning the patties back into the skillet; reduce heat and let them cook through in the broth.
If you prefer a thicker gravy, dissolve flour in a small amount of cold water to make a slurry before whisking it into your simmering broth gradually until desired thickness is achieved.
Once cooked through, serve your Salisbury steak smothered in rich mushroom gravy alongside your favorite sides.