In a large bowl, combine the ground beef, egg, onion-mushroom soup mix, Worcestershire sauce, and breadcrumbs until the mixture reaches a firm consistency and holds together.

Shape the ground meat mixture into round or oval patties, ensuring they are the same size for consistent cooking.

Heat 2 tablespoons of olive oil in a large skillet over medium-high heat. Sear the patties for approximately 4-5 minutes on each side until they develop a golden brown crust. Set the patties aside.

In the same skillet over medium heat, add a tablespoon of oil as well as the sliced mushrooms. Cook them for about 8 minutes or until they soften.

Add the mushroom gravy, adjusting the water quantity for your preferred gravy consistency.

Season the gravy with salt and pepper and then return the browned patties to the skillet, ensuring they are in a single layer and well-coated with the rich gravy.

Reduce the heat to low, cover the skillet, and let it simmer for about 15-20 minutes. This allows the flavors to meld, and the ground beef patties absorb the rich, thickened mushroom gravy.

Ensure the salisbury steaks are thoroughly cooked by cutting them into one to confirm there is no pink in the center. The internal temperature should be at least 160°F (71°C).

Carefully transfer the salisbury steaks onto a serving platter.