• Truss your meat.
• Brush with prepared mustard and season liberally with salt and pepper.
• Sear on all sides in a hot pan then set aside.
• In the same pan, roast flavoring vegetables.
• Deglaze pan with about a half cup of rootbeer.
• Transfer all contents of pan into the slow cooker.
• Set meat on top of the flavoring vegetables.
• Add remaining rootbeer, beef stock, and worcestershire sauce.
• Cook on high for 2 hours.
• When done, remove from stock, tent with foil, and allow to rest.
• Strain cooking liquid into a saucepan and reduce by half.
• Carve meat thinly across the grain.
• Slice french baguette into portions and into top and bottom halves.
• Lay slices of beef on bottom half of bread. Top with cheese and gratinate.
• Top with caramelized onions and assemble sandwiches.
• Serve with dipping sauce.

For the Caramelized Onions

• Slice onions thinly.
• Roast in a slightly oiled pan over high heat. Add a pinch of salt.
• When onions start to caramelize turn heat down and continue stirring.
• Add balsamic vinegar, and continue roasting until dark brown in color.
• Set aside.