Cut a large onion in half. Thinly slice both halves, then finely dice about one-fourth of the onion to make .25 cup. Set the remaining sliced onions aside.
Combine the ground beef, diced onion, panko, egg, salt and pepper in a large bowl. Divide and shape into four oblong patties about .75 inch thick.
Cook the patties in a large skillet over medium heat until browned on both sides.
Remove from skillet and place on a plate. Cover with foil and set aside. They are not fully cooked at this point, just browned.
Add 1 tablespoon of olive oil to the skillet, then add the onion slices. Cook until golden and starting to soften, about 10 minutes. Reduce heat if they start to burn.
Stir in the mushrooms and continue cooking until just about fully cooked. Add the garlic, stir and cook for one additional minute.
Pour in the beef stock and Worschestershire sauce.
Mix the cornstarch with the water to create a slurry. Stir into the skillet to combine. Heat until mixture starts to thicken.
Return the steak patties to the skillet, flip to coat with the mixture, then cover. Cook until the internal temperature of the patties reaches 160 degrees. Serve patties topped with gravy.
Notes
For onion gravy, omit the mushrooms and add an additional cup of thinly sliced onions.
Salisbury steak is traditionally made with ground beef, although you could substitute ground turkey and substitute chicken stock for the beef.