Prepare the Steaks:

In a large bowl, combine the ground beef, breadcrumbs, milk, egg, chopped onion, minced garlic, Worcestershire sauce, mustard powder, salt, and pepper. Mix well until all ingredients are evenly incorporated.
Shape the mixture into 4-6 oval-shaped patties.
Cook the Steaks:

Heat the vegetable oil in a large skillet over medium-high heat.
Add the patties to the skillet and cook for about 4-5 minutes on each side, or until they are browned and cooked through. Remove the patties from the skillet and set them aside.
Make the Gravy:

In the same skillet, add the butter and let it melt.
Add the sliced onion and mushrooms, and cook until they are softened, about 5-7 minutes.
Sprinkle the flour over the onions and mushrooms, and stir well to combine. Cook for another 1-2 minutes to remove the raw flour taste.
Gradually pour in the beef broth, stirring constantly to prevent lumps.
Add the Worcestershire sauce and Dijon mustard. Stir well and bring the mixture to a simmer.
Cook the gravy for about 5-7 minutes, or until it thickens. Season with salt and pepper to taste.
Combine and Serve:

Return the cooked patties to the skillet, spooning some of the gravy over them.
Reduce the heat to low and let the patties simmer in the gravy for about 10 minutes, allowing the flavors to meld together.
Serve:

Serve the Salisbury steaks hot, topped with the rich onion and mushroom gravy.
Enjoy your mouthwatering Salisbury steak!