In a large bowl, combine ground beef, onion, breadcrumbs, egg, 2 tablespoons Worcestershire sauce, Dijon mustard, garlic powder, 1/2 teaspoon black pepper, and 1/2 teaspoon salt. Gently mix until just combined, avoiding overmixing to prevent tough steaks.

Divide the mixture into 6-8 oval patties, about 1/2 to 3/4 inch thick. Make a slight indentation in the center of each patty for even cooking.

Heat 2 tablespoons olive oil in a large skillet over medium-high heat. Sear patties for 3-4 minutes per side until browned, working in batches if necessary. Remove and set aside.

If using, add 1 tablespoon olive oil to the skillet and sauté sliced mushrooms for 5-7 minutes until softened and lightly browned, scraping up any browned bits. Remove and set aside with patties.

Reduce heat to medium-low and sprinkle flour into the skillet. Stir constantly for 1-2 minutes to create a light brown roux, which will help thicken the gravy.

Gradually whisk in the beef broth until all is incorporated and no lumps remain. Stir in the remaining 1 tablespoon Worcestershire sauce, soy sauce, and dried thyme.

Bring the gravy to a gentle simmer, stirring occasionally, until it thickens to your desired consistency (about 5-7 minutes). Add more broth or water if it gets too thick.

Return the seared Salisbury steak patties and cooked mushrooms (if using) to the skillet, nestling them into the simmering gravy.

Reduce heat to low, cover the skillet, and cook for 10-15 minutes, or until cooked through and tender. Alternatively, transfer to a baking dish and bake at 350°F (175°C) for 10-15 minutes if your skillet is not oven-safe.

Taste the gravy and adjust salt and pepper as needed. Serve hot, spooning plenty of gravy over each Salisbury steak.