Heat a large skillet over medium-high heat. Add the butter, then stir in the mushrooms and onions. Sauté for about 5 to 7 minutes, or until the veggies are golden and fragrant.

Add the ground beef and Montreal steak seasoning to the skillet. Break up the meat with a spatula and cook until browned and cooked through, about 5 minutes. Drain any excess fat if necessary.

Sprinkle the flour over the meat and vegetables. Stir well to coat everything evenly, and cook for 1 minute to remove the raw flour taste.

Slowly pour in the chicken broth, stirring constantly to avoid lumps. Add the steak sauce and Dijon mustard, and bring to a gentle simmer.

Let the mixture cook for 4 to 5 minutes, stirring occasionally, until the sauce thickens and becomes glossy.

Serve the beef mixture hot on your favorite buns, with extra sauce spooned on top if desired.