Preheat the oven to 350° Fahrenheit and grease a 9x13-inch baking sheet. Place the bottom half of the rolls in the bottom of the pan.
Caramelize onions: Add 2 tablespoons of butter to a large skillet over medium heat. Add onion and water. Cook onions, stirring occasionally, until they begin to soften and caramelize, about 20 to 25 minutes. Remove from heat.
Prepare French Dip sandwiches: Spread an even layer of stone ground mustard onto rolls. Top with a layer of roast beef and a layer of provolone. Top with caramelized onions, then cover with the tops of the rolls.
Melt 1 tablespoon of butter and mix with Worcestershire, mayonnaise, Dijon mustard, and sesame seeds. Spread over the buns. Cover with foil and bake until golden and cooked through, about 20-30 minutes. Remove from the oven and sprinkle with fresh parsley.
Prepare the Au Jus Sauce: Add remaining butter to a small saucepan over medium heat. Stir in garlic and cook until fragrant, about 1-2 minutes. Add Beef Broth, Worcestershire sauce, thyme, sea salt, and pepper. Simmer for 10 minutes until reduced slightly.
Serve squares of French Dip Sandwiches with Au Jus on the side for dipping.
This nutritional information refers to each serving of the entire recipe and not just the products used as ingredients.