Whisk the beef broth, steak sauce, Dijon mustard, and Worcestershire sauce in a measuring cup. Set it near the stove.

Heat a large skillet over medium-high heat. Add the ground beef and cook for 7-8 minutes, breaking it up, until browned and cooked through. Drain excess grease and transfer the beef to a plate.

Melt the butter in the same skillet. Add the mushrooms, onion, and steak seasoning. Cook for 7-8 minutes, stirring occasionally, until the mushrooms are browned and the onion is soft.

Sprinkle the flour over the mushrooms and onion. Stir constantly for 1 minute until the flour coats the vegetables and smells lightly nutty.

Return the beef to the skillet. Pour in the sauce mixture and stir well. Simmer for 3-5 minutes until the filling is glossy and thick enough to mound on a spoon. Season with salt and pepper as needed.

Pile about 1/3 cup of hot filling onto each toasted bun. Top with shredded cheddar so it softens from the heat, then add pickles or crispy onions if using. Serve right away.