• Preheat oven to 350° and grease a 9"-x-13" baking sheet with cooking spray. Place the bottom half of the rolls in the bottom of the pan.
  12 Buns

To Caramelize Onions

• In a large skillet over medium heat, melt 2 tablespoons of butter. Add onion and 2 sprigs thyme, water (this helps them brown) and season with salt and pepper.
  1/2 Cup Butter, 1 Onion, 4 Sprigs Thyme, 1 teaspoon Water, Kosher Salt, Black Pepper
• Cook, stirring occasionally, until the onions begin to soften and turn slightly golden. Continue cooking, stirring occasionally, until onions are caramelized, 20 to 25 minutes. Remove from heat.

For The Sandwiches

• Spread an even layer of mustard onto bread. Top with a layer of roast beef and a layer of provolone. Top with caramelized onions, then cover with the tops of the rolls.
  3 Tablespoon Dijon mustard, 1/2 Pound Deli Roast Beef, 9-12 Slices Provolone Cheese
• Melt all but one tablespoon of the remaining butter and mix with 1 teaspoon worcestershire, mayonnaise, dijon mustard, and the toasted sesame seeds. Spread over the buns. Cover with foil. Bake until golden and cooked through, about 20-30 minutes. Sprinkle the parsley on top after baking.
  1/2 Cup Butter, 1 teaspoon Worcestershire Sauce, 2 Tablespoons Mayonnaise, 1 teaspoon Dijon Mustard, 1 teaspoon Toasted Sesame Seeds, 1 Tablespoon Parsley

To make the Au Jus

• Meanwhile,  melt remaining 1 tablespoon butter in a medium skillet over medium heat. Stir in garlic and cook until fragrant. Add beef stock, Worcestershire sauce and remaining 2 sprigs thyme, stripped from stem. Season with salt and pepper. Simmer for 10 minutes until reduced slightly. Serve squares with au jus on the side for dipping. 
  1/2 Cup Butter, 4 Sprigs Thyme, Kosher Salt, 1 Clove Garlic, 1 Cup Beef Broth, 1 Tablespoon Worcestershire Sauce, Black Pepper