In a large bowl, combine ground beef, breadcrumbs, egg, ketchup, Worcestershire sauce, onion powder, garlic powder, salt, and black pepper. Mix by hand until just combined—do not overmix.

Shape the mixture into 4 equal oval patties, about 2 cm thick.

Heat the oil in a large skillet over medium-high heat. Add the patties and cook for 4-5 minutes per side, until browned on both sides. Transfer to a plate and set aside.

In the same skillet, reduce heat to medium. Melt the butter and add the onion. Cook for 3-4 minutes until softened.

Add the sliced mushrooms and cook another 5 minutes, stirring occasionally, until they start to brown.

Sprinkle the flour over the onion and mushrooms. Stir and cook for 1 minute.

Gradually whisk in the beef broth, scraping up any browned bits. Stir in Worcestershire sauce, Dijon mustard, salt, and pepper.

Return the patties to the skillet, nestling them into the gravy. Reduce heat to low, cover, and simmer for 15 minutes, occasionally spooning gravy over the steaks.

Check that patties are cooked through (internal temperature of 70°C/160°F). Adjust seasoning if needed.

Serve Salisbury steaks hot, topped with plenty of mushroom onion gravy.