Preheat oven to 350 degrees F. Spray a 9x13-inch baking dish with nonstick cooking spray; set aside.
Add ground beef and onion to a large skillet over medium-high heat and cook for 8-10 minutes, breaking up, until cooked through. Add salt, pepper, Worcestershire sauce and garlic and cook for an additional 1-2 minutes. Drain excess grease and set aside.
To make the mushroom sauce, heat olive oil in a saucepan over medium high heat and sauté mushrooms for 5 minutes. Add beef broth, salt, pepper, Worcestershire sauce and balsamic vinegar, then mix to combine.
Whisk together milk and cornstarch to form a slurry and mix into mushroom mixture. Cook for 1-2 minutes, or until thickened then remove from heat and set aside.
Slice rolls horizontally (keeping them connected) and place the bottom half in the prepared pan. Top with meat mixture, mushroom sauce and sliced cheese. Place the top half of buns over the cheese and brush with melted butter substitute and sprinkle sesame seeds on top.
Bake, uncovered, for 15-20 minutes, or until the tops of the rolls are golden brown and cheese is melted. Remove from the oven, cut into individual sandwiches and serve.