**Prepare the Salisbury Steaks**
- In a mixing bowl, combine ground beef, breadcrumbs, egg, Worcestershire sauce, Dijon mustard, onion powder, garlic powder, salt, and black pepper.
- Mix until just combined, then shape into 4 oval-shaped patties.
**Cook the Patties**
- Heat olive oil in a large skillet over medium-high heat.
- Add the patties and cook for 4-5 minutes per side, until browned.
- Remove from skillet and set aside.
**Make the Mushroom Gravy**
- In the same skillet, melt butter over medium heat.
- Add onions and mushrooms, cooking until softened (about 5 minutes).
- Sprinkle flour over the mushrooms and stir well.
- Gradually add beef broth while whisking to prevent lumps.
- Stir in Worcestershire sauce, Dijon mustard, salt, pepper, and thyme.
- Simmer for 5 minutes until thickened.
**Simmer the Steaks in Gravy**
- Return the Salisbury steaks to the skillet, spooning gravy over them.
- Simmer for 10 minutes, allowing the flavors to meld.
**Serve**
- Serve hot with mashed potatoes, rice, or buttered noodles. Enjoy!