Preheat griddle or panini press to medium low heat.
Prepare Au Jus according to packet instructions.
Melt 2 tablespoons of the butter in a skillet over medium heat. Add in the onions and reduce the heat to medium low. Cook, stirring occasionally, until lightly caramelized,about 25-30 minutes.
In a large bowl, mix the shaved beef with the beef broth and Worcestershire sauce. Season with salt and pepper.
Add the beef to the skillet full of onions Increase heat to medium-high and brown the beef. Once browned, drain off excess grease.
Evenly distribute the beef and onion mixture over the bottom the Mini Sub Buns. Top with the gruyere cheese. Cover with the top buns.
In a griddle or panini press, over medium low heat, melt remaining butter. Add sandwiches, two at a time, and cook until bread is golden brown and cheese has completely melted. Repeat with remaining two sandwiches and serve immediately.
Serve and dip your French Onion Beef Panini in Au Jus and enjoy!